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Creation of an ODOT specification for patching or overlay of bridge decks.

CREATION OF AN ODOT SPECIFICATION FOR
PATCHING OR OVERLAY OF BRIDGE DECKS
FINAL REPORT - FHWA-OK-08-09
ODOT SPR ITEM NUMBER 2184
By
Chris C. Ramseyer
Assistant Professor
Daniel S. Myers
Research Assistant
Civil Engineering and Environmental Science
University of Oklahoma
Norman, Oklahoma
March 2009 TECHNICAL REPORT DOCUMENTATION PAGE
1. REPORT NO. FHWA-OK-08-09
2. GOVERNMENT ACCESSION NO.
3. RECIPIENT=S CATALOG NO.
4. TITLE AND SUBTITLE CREATION of an ODOT Specification for Patching or Overlay of Bridge Decks
5. REPORT DATE September 2009
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S) Chris C. Ramseyer and Daniel S. Myers
8. PERFORMING ORGANIZATION REPORT
9. PERFORMING ORGANIZATION NAME AND ADDRESS University of Oklahoma 202 w. Boyd, room 334 Norman, Oklahoma 73019
10. WORK UNIT NO.
11. CONTRACT OR GRANT NO. ODOT Item Number 2184
12. SPONSORING AGENCY NAME AND ADDRESS Oklahoma Department of Transportation Planning and Research Division 200 N.E. 21st Street, Room 3A7 Oklahoma City, OK 73105
13. TYPE OF REPORT AND PERIOD COVERED Final Report From October 2004 To October 2008
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16 . ABSTRACT Bridge deck cracking is a huge problem in the United States, and various agencies have sponsored research endeavoring to determine the underlying problems. A number of causes have been identified, including thermal movement, plastic shrinkage, and early age settlement, as well as a number of other issues. Polymer fibers are a possible solution to many of the causes of bridge deck cracking: they have been shown to help early age properties like shrinkage and movement, and as a bonus, fibers improve post-cracking behavior. More understanding of the benefits and uses of polymer fibers in concrete is needed. This study researched the properties of four polymer fibers; two of the fibers were macrofibers, and two were microfibers. Each fiber was tested at several dosage rates to identify optimum dosage levels. Early age shrinkage, long term shrinkage, compressive strength, and tensile strength were investigated. Macrofibers and microfibers were found to have different impacts on concrete behavior, with different optimal dosage rates. Microfibers greatly dried out the concrete mixture, hindering workability. However, the microfibers substantially reduced plastic shrinkage and improved concrete strength at early age. Macrofibers, while not hindering workability, did not provide benefits as great as the microfibers to the concrete strength.
17. KEY WORDS Very Early Strength Concrete, Accelerator, Rapid Strength Concrete, Patching, Concrete Repair
18. DISTRIBUTION STATEMENT No restrictions. This publication is available from the Planning & Research Division, Oklahoma DOT.
19. SECURITY CLASSIF. (OF THIS REPORT) Unclassified
20. SECURITY CLASSIF. (OF THIS PAGE) Unclassified
21. NO. OF PAGES 98
22. PRICE N/A
ii SI (METRIC) CONVERSION FACTORS
Approximate Conversions to SI Units
Approximate Conversions from SI Units
Symbol When you know Multiply by To Find Symbol
Symbol When you know Multiply by To Find Symbol
LENGTH in inches 25.40 millimeters mm ft feet 0.3048 meters m yd yards 0.9144 meters m mi miles 1.609 kilometers km AREA in² square inches 645.2 square millimeters mm ft² square feet 0.0929 square meters m² yd² square yards 0.8361 square meters m² ac acres 0.4047 hectares ha mi² square miles 2.590 square kilometers km² VOLUME fl oz fluid ounces 29.57 milliliters mL gal gallons 3.785 liters L ft³ cubic feet 0.0283 cubic meters m³ yd³ cubic yards 0.7645 cubic meters m³
LENGTH mm millimeters 0.0394 inches in m meters 3.281 feet ft m meters 1.094 yards yd km kilometers 0.6214 miles mi AREA mm² square millimeters 0.00155 square inches in² m² square meters 10.764 square feet ft² m² square meters 1.196 square yards yd² ha hectares 2.471 acres ac km² square kilometers 0.3861 square miles mi² VOLUME mL milliliters 0.0338 fluid ounces fl oz L liters 0.2642 gallons gal m³ cubic meters 35.315 cubic feet ft³ m³ cubic meters 1.308 cubic yards yd³
MASS oz ounces 28.35 grams g lb pounds 0.4536 kilograms kg T short tons 0.907 megagrams Mg (2000 lb) TEMPERATURE (exact) ºF degrees (ºF-32)/1.8 degrees ºC Fahrenheit Celsius FORCE and PRESSURE or STRESS lbf poundforce 4.448 Newtons N lbf/in² poundforce 6.895 kilopascals kPa per square inch
MASS g grams 0.0353 ounces oz kg kilograms 2.205 pounds lb Mg megagrams 1.1023 short tons T (2000 lb) TEMPERATURE (exact) ºC degrees 9/5+32 degrees ºF Celsius Fahrenheit FORCE and PRESSURE or STRESS N Newtons 0.2248 poundforce lbf kPa kilopascals 0.1450 poundforce lbf/in² per square inch
iii The contents of this report reflect the views of the author(s) who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the views of the Oklahoma Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. While trade names may be used in this report, it is not intended as an endorsement of any machine, contractor, process, or product. CREATION OF AN ODOT SPECIFICATION FOR PATCHING
OR OVERLAY OF BRIDGE DECKS
FINAL REPORT
By
DANIEL S. MYERS
Research Assistant
Under the Supervision of
Chris C. Ramseyer, Ph.D., P.E.
Assistant Professor
Civil Engineering and Environmental Science
University of Oklahoma
202 W. Boyd, room 334
Norman, Oklahoma 73019
March 2009
v Table of Contents
Acknowledgements
..........................................................................................
iv
List of Tables
...................................................................................................
xii
List of Figures
................................................................................................
xiii
Abstract
........................................................................................................
xviii
Chapter 1
: Introduction
............................................................................
20
Chapter 2
: Literature Review
..................................................................
21
2.1
Bridge Deck Cracking
...............................................................................
21
2.1.1
Scope of the Problem
.....................................................................
22
2.1.2
Mechanics of Cracking
..................................................................
24
2.1.3
Shrinkage
.....................................................................................
31
2.1.3.1
Plastic (Early Age) Shrinkage
...............................................
31
2.1.3.2
Autogenous Shrinkage
..........................................................
38
2.1.3.3
Drying (Long Term) Shrinkage
............................................
43
2.1.3.4
Carbonation Shrinkage
..........................................................
47
2.1.4
Thermal Effects
............................................................................
47
2.1.4.1
Heat of Hydration
.................................................................
48
2.1.4.2
Temperature at Casting
.........................................................
50
2.1.4.3
Cooling After Batching
.........................................................
52
2.1.4.4
Diurnal Cycle
........................................................................
53
2.1.4.5
Annual Cycle
........................................................................
53
vi 2.1.4.6
Solar Radiation Heat
.............................................................
54
2.1.4.7
Compared with temperature at casting
.................................
57
2.1.4.8
Coefficient of Thermal Expansion
........................................
57
2.1.5
Deflections
...................................................................................
59
2.1.5.1
Live Loads
............................................................................
59
2.1.5.2
Formwork
..............................................................................
59
2.1.6
Restraint
.......................................................................................
60
2.1.6.1
Internal
..................................................................................
60
2.1.6.2
External
.................................................................................
63
2.1.6.3
Expansion joints
....................................................................
66
2.1.7
Modulus of elasticity
....................................................................
66
2.1.7.1
Modulus gain
........................................................................
67
2.1.8
Creep of Concrete
........................................................................
68
2.1.8.1
Mix Design
............................................................................
70
2.1.8.2
Curing Conditions
.................................................................
71
2.1.8.3
Admixtures
............................................................................
71
2.1.8.4
Plastic Settlement
..................................................................
71
2.1.9
Geometry
......................................................................................
71
2.1.9.1
Skew
......................................................................................
72
2.1.9.2
Depth of Deck
.......................................................................
72
2.1.9.3
Cover
.....................................................................................
72
2.1.10
Tensile Strength
...........................................................................
73
2.1.10.1
Fibers
.....................................................................................
75
vii
2.1.10.2
Mix Design
............................................................................
75
2.1.11
Corrosion
......................................................................................
75
2.1.11.1
Chloride Ion Penetration
.......................................................
76
2.1.11.2
Rebar Type
............................................................................
76
2.1.12
Department of Transportation Opinions
......................................
77
2.1.13
Application in the Field
...............................................................
86
2.1.14
Summary/Conclusion
...................................................................
88
2.2
Fiber-Reinforced Concrete
.....................................................................
92
2.2.1
Fiber Material Properties
.............................................................
92
2.2.2
Workability
..................................................................................
94
2.2.3
Early Age Shrinkage
....................................................................
95
2.2.4
Long Term Shrinkage
..................................................................
96
2.2.5
Compression Strength
..................................................................
97
2.2.6
Tensile Strength
...........................................................................
99
2.2.7
Flexure
.........................................................................................
100
2.2.8
Modulus of Elasticity
...................................................................
101
2.2.9
Failure Types
...............................................................................
101
2.2.10
Fibers as Crack Inhibitors
............................................................
102
2.2.10.1
Crack Width and Time to Cracking
......................................
102
2.2.10.2
Impact Resistance
.................................................................
103
2.2.11
Fiber-Reinforced Concrete: Conclusion
......................................
103
2.3
Literature Review: Conclusion
...............................................................
104
Chapter 3
: Research Scope
.....................................................................
106
viii
3.1
Tests
...........................................................................................................
107
3.1.1
Fresh Concrete Tests
....................................................................
107
3.1.2
Compression Strength
..................................................................
109
3.1.3
Tensile Strength
...........................................................................
110
3.1.4
Unrestrained Shrinkage
................................................................
111
3.1.5
Unrestrained Shrinkage from Time Zero
.....................................
112
3.2
Matrix
........................................................................................................
116
3.3
Fibers
.........................................................................................................
118
3.3.1
Stealth
..........................................................................................
118
3.3.2
Grace Microfiber
........................................................................
119
3.3.3
Strux 90/40
.................................................................................
120
3.3.4
High Performance Polymer (HPP)
.............................................
121
3.4
Base Mix
..................................................................................................
122
3.5
Typical Batching Procedure
..................................................................
123
3.5.1
Pre-batching preparation
............................................................
123
3.5.2
Batching Procedure
....................................................................
125
Chapter 4
: Results
.................................................................................
127
4.1
Fresh Concrete Tests and Conditions
..................................................
128
4.2
Compression Tests
.................................................................................
131
4.3
Splitting Tensile Tests
............................................................................
134
4.4
Unrestrained Shrinkage
........................................................................
136
4.5
Unrestrained Shrinkage from Time Zero
............................................
138
4.6
Fiber-Reinforced Concrete: Summary of Results
..............................
140
ix
4.6.1
Stealth Summary
........................................................................
140
4.6.2
Grace Microfiber Summary
.......................................................
141
4.6.3
Strux 90/40 Summary
................................................................
141
4.6.4
HPP Summary
............................................................................
142
Chapter 5
: Discussion
...........................................................................
143
5.1
Workability
.............................................................................................
143
5.1.1
Slump
.........................................................................................
144
5.1.2
Finishing
....................................................................................
146
5.2
Fresh Concrete Characteristics
............................................................
147
5.3
Shrinkage
................................................................................................
149
5.3.1
The Unrestrained Shrinkage from Time Zero Test
....................
150
5.3.2
Shrinkage from Time Zero
.........................................................
157
5.3.2.1
Shrinkage from Time Zero: Stealth
....................................
157
5.3.2.2
Shrinkage from Time Zero: Grace Microfiber
....................
158
5.3.2.3
Shrinkage from Time Zero: Strux 90/40
.............................
159
5.3.2.4
Shrinkage from Time Zero: HPP
........................................
160
5.3.3
ASTM Unrestrained Shrinkage
..................................................
161
5.4
Plain Concrete Control Mixes
...............................................................
163
5.4.1
Plain Concrete: Fresh Concrete Properties
................................
164
5.4.2
Plain Concrete: Shrinkage from Time Zero
...............................
165
5.4.3
Plain Concrete: ASTM Unrestrained Shrinkage
........................
166
5.4.4
Plain Concrete: Compression Strength
......................................
167
5.4.5
Plain Concrete: Splitting Tensile Strength
.................................
167
x
5.5
Fiber Evaluation
.....................................................................................
169
5.5.1
General Survey of Fibers
...........................................................
169
5.5.1.1
Unrestrained Shrinkage from Time Zero
............................
169
5.5.1.2
ASTM Unrestrained Shrinkage
...........................................
170
5.5.1.3
Compression Strength
.........................................................
171
5.5.1.4
Splitting Tensile Strength
...................................................
174
5.5.2
Stealth Optimum Dosage
...........................................................
176
5.5.3
Grace Microfiber Optimum Dosage
..........................................
179
5.5.4
Strux 90/40 Optimum Dosage
...................................................
181
5.5.5
HPP Optimum Dosage
...............................................................
185
5.6
Microfiber and Macrofiber comparison
..............................................
188
5.7
Impact of Fibers: Summary
..................................................................
189
Chapter 6
: Conclusions
.........................................................................
191
References
......................................................................................................
193
xi
List of Tables
Table 1
: Restrained shrinkage stresses and age at cracking (Altoubat and Lange, 2002)
.....................................................................................................................
74
Table 2: Causes of bridge deck cracking, agency survey (Krauss and Rogalla, 1996)
...............................................................................................................................
78
Table 3
: Factors affecting bridge deck cracking (Krauss and Rogalla, 1996)
............
79
Table 4
: Probability of Plastic Shrinkage Cracking (Babaei, 2005)
...........................
84
Table 5
: Primary matrix
..............................................................................................
117
Table 6
: Primary matrix testing regimen
....................................................................
117
Table 7
: Base mix
.....................................................................................................
123
Table 8
: Primary matrix batches
...............................................................................
127
Table 9
: Primary matrix batching conditions
............................................................
128
Table 10
: Primary matrix fresh concrete properties
................................................
130
Table 11
: Primary matrix compression test results
...................................................
133
Table 12
: Primary matrix splitting tensile strength
...................................................
136
Table 13
: ASTM unrestrained shrinkage test results (normalized at 1 day)
.............
137
Table 14: Unrestrained shrinkage from time zero tests results (normalized at time 0)
.............................................................................................................................
139
Table 15
: Plain concrete fresh concrete properties and batch conditions
.................
164
Table 16
: Macrofiber and microfiber comparison
....................................................
189
Table 17
: General impact of fibers
...........................................................................
190
xii
List of Figures
Figure 1
: Causes of bridge deck cracking (Brown, et al., 2001)
..................................
24
Figure 2
: Time dependence of restrained shrinkage and creep (Brown et al., 2001
after Mehta, 1993)
...................................................................................................
27
Figure 3
: Factors affecting cracking in bridge decks
..................................................
30
Figure 4
: Accumulation of early age and long term shrinkage, with various curing
environments during the first day. Wind = 2 m/s (4.5mph), dry = 40% RH, wet =
100% RH. (Holt, 2001)
.........................................................................................
34
Figure 5
: Reactions causing autogenous and chemical shrinkage (Holt, 2001 from
Japan, 1999)
..........................................................................................................
39
Figure 6
: Schematic of a cross-section of hydrating cement paste (Jensen and Hansen,
2000). Left: low degree of hydration. Right: high degree of hydration
.............
40
Figure 7
: Direction of shift in early age autogenous shrinkage when influenced by
other factors (Holt, 2001)
......................................................................................
42
Figure 8
: Strain effects of various temperature changes (Krauss and Rogalla, 1996) 56
Figure 9
: Example deck and steel girder stresses for various temperature changes
(Krauss and Rogalla, 1996)
...................................................................................
57
Figure 10
: Time dependence of restrained shrinkage, creep, and tensile strength
(Brown et al., 2001 after Mehta, 1993)
.................................................................
70
Figure 11
: Time dependence of restrained shrinkage, stress relaxation (creep), and
tensile strength (Brown et al., 2001 after Mehta, 1993)
.......................................
74
Figure 12
: Frequency of top three causes of early-age bridge deck cracking (Aktan et
al., 2003)
...............................................................................................................
83
xiii
Figure 13
: Factors affecting cracking in bridge decks: level of engineer control
......
89
Figure 14
: Effect of paste volume fraction on workability of steel fiber-reinforced
mortars with 30 mm fibers (Johnston, after Pfeiffer and Soukatchoff, 1994)
......
94
Figure 15
: Air content pressurized air pot apparatus
..................................................
108
Figure 16
: Slump test apparatus
..................................................................................
108
Figure 17
: Compression test with Forney compression testing machine
...................
110
Figure 18
: Splitting tensile test
...................................................................................
111
Figure 19
: Unrestrained shrinkage test
.......................................................................
112
Figure 20
: Unrestrained shrinkage from time zero test in progress
............................
113
Figure 21
: Time zero molds prepared for filling
........................................................
114
Figure 23
: Stealth Microfibers
..................................................................................
119
Figure 24
: Grace Microfibers
....................................................................................
120
Figure 22
: Strux 90/40 Fibers
...................................................................................
121
Figure 25
: HPP (High Performance Polymer) fibers
................................................
122
Figure 26
: Coarse aggregate pile
..............................................................................
124
Figure 27
: Batching area
...........................................................................................
125
Figure 28
: Environmental chamber and samples: A – 4x8” cylinders, B – unrestrained
shrinkage from time zero samples, C – ASTM unrestrained shrinkage samples, D
–
restrained ring tests (not used in this research)
................................................
126
Figure 29
: Plain concrete compression failure: brittle
..............................................
131
Figure 30
: Fiber-reinforced concrete compression failure: ductile (Strux 90/40 10lb
dosage)
................................................................................................................
132
Figure 31
: Fiber-reinforced concrete splitting tensile failure: ductile
......................
135
xiv
Figure 32
: Slump versus fiber dosage
.......................................................................
144
Figure 33
: Concrete mixture with high dosage of HPP fibers
..................................
145
Figure 34
: Concrete finish on HPP high dosage mix at time of casting
...................
146
Figure 35
: Concrete finish on Strux 90/40 high dosage mix after unmolding
..........
147
Figure 36
: Unit weight versus fiber dosage
..............................................................
148
Figure 37
: Air content versus fiber dosage
...............................................................
149
Figure 38
: Time Zero Mold Comparisons to 24 hours (Strux 90/40 1lb)
.................
150
Figure 39
: Comparison of Time Zero Molds from 4 hours (Strux 90/40 1lb)
..........
151
Figure 40
: Average time zero versus average ASTM unrestrained shrinkage
.........
152
Figure 41
: Time Zero versus ASTM Unrestrained Shrinkage
..................................
153
Figure 42
: Time Zero versus ASTM Unrestrained Shrinkage (Strux 90/40 high
dosage rates)
........................................................................................................
154
Figure 43: Time Zero versus ASTM Unrestrained Shrinkage (HPP high dosage rates)
.............................................................................................................................
154
Figure 44
: Accumulation of early age and long term shrinkage, with various curing
environments during the first day. Wind = 2 m/s (4.5mph), dry = 40% RH, wet = 100% RH. (Holt, 2001)
.......................................................................................
156
Figure 45
: Time zero shrinkage results: Stealth
.......................................................
158
Figure 46
: Time zero shrinkage results: Grace Microfiber
.......................................
159
Figure 47
: Time zero shrinkage results: Strux 90/40
................................................
160
Figure 48
: Time Zero shrinkage results: HPP
...........................................................
161
Figure 49
: Unrestrained shrinkage at 28 days (bars show data range)
.....................
162
Figure 50
: Unrestrained shrinkage curves: HPP
.......................................................
163
xv
Figure 51
: Plain concrete shrinkage from time zero
.................................................
165
Figure 52
: Plain concrete ASTM unrestrained shrinkage (bars show data range)
...
166
Figure 53
: Plain concrete compression strength (bars show data range)
..................
167
Figure 54
: Plain concrete splitting tensile strength (error bars show data range)
.....
168
Figure 55
: Unrestrained shrinkage from time zero: 24 hour readings
......................
170
Figure 56
: ASTM unrestrained shrinkage at 28 days (bars show data range)
..........
171
Figure 57
: Compression strength at 24 hours (bars show data range)
......................
172
Figure 58
: Compression strength at 28 days (bars show data range)
.......................
173
Figure 59
: Splitting tensile strength at 24 hours (bars show data range)
..................
174
Figure 60
: Splitting tensile strength at 28 days (bars show data range)
...................
176
Figure 61
: Stealth slump versus fiber dosage and shrinkage at 24 hours
.................
177
Figure 62
: Stealth strength at 24 hours
.....................................................................
177
Figure 63
: Stealth strength at 28 days
.......................................................................
178
Figure 64
: Grace Microfiber slump versus fiber dosage and shrinkage at 24 hours 179
Figure 65
: Grace Microfiber 24 hour strengths
........................................................
180
Figure 66
: Grace Microfiber 28 day strengths
..........................................................
181
Figure 67
: Strux 90/40 slumps
..................................................................................
182
Figure 68
: Strux 90/40 plastic shrinkage
..................................................................
183
Figure 69
: Strux 90/40 24 hour strength
...................................................................
184
Figure 70
: Strux 90/40 28 day strength
....................................................................
185
Figure 71
: HPP slumps
.............................................................................................
186
Figure 72
: HPP plastic shrinkage results
..................................................................
186
Figure 73
: HPP 24 hour strengths
.............................................................................
187
xvi
Figure 74: HPP 28 day strength................................................................................ 188
xvii
Abstract
Bridge deck cracking is a huge problem in the United States, and various agencies have sponsored research endeavoring to determine the underlying problems. A number of causes have been identified, including thermal movement, plastic shrinkage, and early age settlement, as well as a number of other issues. Polymer fibers are a possible solution to many of the causes of bridge deck cracking: they have been shown to help early age properties like shrinkage and movement, and as a bonus, fibers improve post-cracking behavior. More understanding of the benefits and uses of polymer fibers in concrete is needed.
This study researched the properties of four polymer fibers; two of the fibers were macrofibers, and two were microfibers. Each fiber was tested at several dosage rates to identify optimum dosage levels. Early age shrinkage, long term shrinkage, compressive strength, and tensile strength were investigated.
Macrofibers and microfibers were found to have different impacts on concrete behavior, with different optimal dosage rates. Microfibers greatly dried out the concrete mixture, hindering workability. However, the microfibers substantially reduced plastic shrinkage and improved concrete strength at early age. Macrofibers, while not hindering workability, did not provide benefits as great as the microfibers to the concrete strength.
xviii
In general, several key results were identified, and it is suggested that many of these impacts can be explained by considering that the polymer fibers have a modulus of elasticity well below that of the hardened concrete matrix. Fibers were found to greatly reduce early age shrinkage, with the effect increasing with increasing dosage levels. Long term shrinkage is not affected by the addition of polymer fibers. Early age concrete strength is improved with the addition of fibers, but long term strength is sometimes reduced with high dosages of fibers. It is noted that these characteristics of polymer fibers indicate that they will be very useful in combating the bridge deck cracking problem.
xix
Chapter 1: Introduction
Bridge decks have many problems with cracking. More than 100,000 bridge decks, nearly half of the bridges in the United States, showed transverse cracking at early age (Krauss and Rogalla, 1996). Early age cracking is the most common deck distress reported by the State Highway Agencies. In all , 97% of state Departments of Transportation indicated that they have problems with early age cracking (Aktan et al., 2003).
Numerous studies have been performed on these problems, and several of the primary causes have been isolated. These include thermal movement, early age shrinkage, and early age settlement (Krauss and Rogalla, 1996; Babaei, 2005). These causes may all be counteracted by the addition of polymer fibers. Polymer fibers have been shown to be beneficial to the early age properties of concrete, as well as to crack mitigation (Kao, 2005).
Research presented here analyzes a number of fibers and dosage rates for their strength and shrinkage properties. Four types of fibers are tested; each one is tested at three to five different dosage rates. The results indicate that long term strength is not strongly impacted by polymer fiber addition, but early age shrinkage is greatly decreased and early age strength is increased.
20
Chapter 2: Literature Review
There has been considerable research work done on both ends of the field: bridge deck cracking and fiber reinforcement. General reviews of the bridge deck cracking problem have been conducted by the National Cooperative Highway Research Program (NCHRP) and several Departments of Transportation (DOT’s). These reviews analyze the problems statistically, and provide a summary of many variables important to the problem. Fiber reinforcement has typically been regarded as a simple crack reducer, but there is research investigating many aspects of its impact on material properties. Fibers impact the bridge deck cracking problem on several fronts, not simply by bridging cracks. A review of research done on both bridge deck cracking and fiber-reinforced concrete is presented here.
2.1 Bridge Deck Cracking
Bridge deck cracking is a problem throughout the United States, as several surveys indicate. A number of state departments of transportation, including Michigan, Texas, Oregon, Utah, New Jersey, Minnesota and Colorado have launched studies on the problem (Brooks, 2000; Brown et al., 2001; Xi et al., 2003; Aktan et al., 2003; Linford and Reaveley, 2004), and in 1996 NCHRP conducted a major project entitled “Transverse Cracking in Newly Constructed Bridge Decks”. This project, undertaken by Krauss and Rogalla, was a comprehensive analysis of the cracking problem at that point, and set out the problems in great detail. Since then a number of projects have conducted research according to the recommendations of that report. The departments of transportation performed similar analyses, researching the problem
21
statistically through surveys, and then identifying the primary causes of cracking. Applicable laboratory research and extensive field studies on new bridges were done to test various methods of mitigating the problem.
An interesting aspect of the present cracking problem is that it has increased as the strength of the concretes used has increased. This may indicate that something about the newer high-performance concretes encourages cracking, unless some other variable such as workmanship or curing is becoming worse during the same period of time. This literature review will investigate why that may be, and what to do about it (Xi et al., 2003).
2.1.1 Scope of the Problem
A large proportion of the bridges in the United States crack at early age. Aktan et al. (2003) found that early age cracking is the single most prevalent deck distress reported by the State Highway Agencies. More than 100,000 bridge decks in the United States showed transverse cracking at early age, according to Krauss and Rogalla (1996); this is nearly half of the bridges. Their survey included 52 DOT’s in the United States and Canada. Sixty-two percent of these agencies considered transverse cracking a problem; fifteen percent believed that all of their bridges suffered from transverse cracking. The respondents stated that, on average, forty-two percent of bridge decks cracked in the first week.
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In the report for the Utah Department of Transportation (Linford and Reaveley, 2004) a database of 71 newly-constructed bridges in the I-15 reconstruction project was created. The bridges were constructed between April 1998 and March 2001. The bridges were each ranked with a Cracking Severity Index Number (CSIN). Cracking was found on 70 of the 71 bridges. Diagonal cracking was found on 87% of the bridges, primarily near abutments or interior bents. Transverse cracking was found on 67% of the bridges; according to the report, this, they postulate, was caused by concrete shrinkage. Only 11% have visible longitudinal cracks.
The report for the Colorado Department of Transportation (Xi et al., 2003) analyzed 72 structures built between 1993 and 2000. These were inspected in 2002. At that time, 82% of the bridges had deck cracking. In addition, the report declared that the Nevada Department of Transportation stated that 75% of all new bridges have a significant cracking problem. The Kansas Department of Transportation indicated that their cracking problems have been mostly resolved; they attribute their success to the implementation of a wet burlap 7 day curing procedure, which cut deck cracking by 50%.
Michigan conducted a survey of the state Departments of Transportation in 2002 (Aktan et al., 2003). Thirty-one states responded. Of these, 97% indicated that they had an early age cracking problem in reinforced concrete bridge decks. Nearly all of those first observed bridge deck cracking within the first year, and most within the first few months. Seventy-eight percent of the respondents stated that transverse
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cracking was the most prevalent, with 16% citing longitudinal cracks, and 6% diagonal.
2.1.2 Mechanics of Cracking
Krauss and Rogalla (1996) carefully considered the mechanics of the cracking problem in their report on transverse cracking in bridge decks. Concrete bridge decks develop cracks when the tensile stress in the concrete exceeds the tensile strength of the concrete at that time. The tensile stresses come from concrete shrinkage, temperature changes in the concrete, and sometimes from self-weight or traffic loads. The stresses develop in the bridge decks because the girders restrain the natural thermal and shrinkage movement of the deck, thus translating the strain into stress.
Brown et al. (2001) endeavored to further isolate the mechanical causes of bridge deck cracking. Figure 1 shows the flow chart they created showing the primary factors in the cracking problem.
Figure 1: Causes of bridge deck cracking (Brown, et al., 2001)
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As seen in this figure, Brown et al. consider shrinkage, thermal stresses, and restraint to be the primary factors in cracking. Later in this literature review, each of these factors will be considered in greater detail.
Shrinkage of concrete is a primary source of strain in bridge decks, and can produce enough strain to crack concrete without additional strain from temperature sources (Krauss and Rogalla, 1996). It is considered by many to be the greatest culprit in the cracking problem (Krauss and Rogalla, 1996).
Temperature effects are the other important source of strain in the concrete matrix. The concrete sets at a specific temperature, locking the matrix to zero temperature stress at that temperature. However, the deck changes temperature, seasonally, daily, from cooling off after the heat of hydration subsides, and from solar radiation on the top surface. These four sources cause significant temperature movement, which occurs according to the coefficient of thermal expansion of the concrete. The stresses induced can both be high and significantly non-uniform (Krauss and Rogalla, 1996).
The final sources of strain are the dead loads and live loads on the structure, along with formwork deflection issues. These strains are less significant, but of concern nonetheless. Several state departments of transportation considered these to be a source of cracking (Krauss and Rogalla, 1996).
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In an unrestrained system, strain does not cause cracking, but when the system is restrained, the strain translates to stress and causes cracking. The restraint of the deck’s movement converts the strain into stress, according to the modulus of elasticity. Both external and internal sources can provide the restraint. The chief external source is the girders that the deck rests upon. Since the girders will not shrink at the same rate as the deck unless they are cast at the same time of the same material, the girders restrain the deck’s movement. In addition, material differences can cause differential restraint of temperature movements. Internally, rebar, aggregate, and fibers are some of the sources of restraint (Krauss and Rogalla, 1996).
There are several other factors that influence the mechanical cracking problem. Stress relaxation or “creep” of concrete is another key issue, as it is the one factor that can reduce the stresses on the concrete. Altoubat and Lange (2002) analyzed this factor in considerable detail. They found that creep can reduce shrinkage stresses by 50% (depending on the mix design), thus doubling the strain capacity at failure.
Krauss and Rogalla (1996) consider the modulus of elasticity to be another important factor in the cracking problem. The modulus of elasticity of the concrete determines the rate of conversion from strain to stress. Therefore, the stress in the concrete will be higher with a higher modulus of elasticity given the same strain conditions.
The geometry of the bridge deck and girders can also have significant impacts on the cracking behavior of the concrete. Krauss and Rogalla (1996) analyzed different
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designs analytically and found that the geometry of the deck significantly impacted the shrinkage and thermal strain fields.
Corrosion of reinforcing steel is a well known factor; however, it typically does not become important for several years. Since the present cracking problems usually show up within a year, the corrosion issue will only be considered in passing.
The final factor in the cracking process is the tensile strength of the concrete itself. After the stresses are created by the factors mentioned previously, whether the concrete finally cracks or not is determined by comparing the stress to the tensile strength of the concrete. As shown in Figure 2, both the stress and the tensile strength of the concrete change with time and it is when the stress finally exceeds the tensile strength of the concrete that cracking occurs (Brown et al., 2001 after Mehta, 1993).
Figure 2: Time dependence of restrained shrinkage and creep (Brown et al., 2001 after Mehta , 1993)
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In summary, the literature indicates that the mechanical process that creates the cracking is as follows. Shrinkage and thermal movement, along with deflections to some extent, put a strain on the deck. This strain would cause no stress if it was unrestrained, but restraint is provided both by the girders and by the reinforcement. This restraint converts some 80% of the strain to stress, depending on the degree of restraint. The actual conversion rate is the modulus of elasticity of the concrete. The creep of the concrete reduces stress by a significant but hard to quantify amount. This stress field is modified by the geometry of the deck, and finally the stress and the tensile strength of the concrete may be compared to see whether cracking is likely to occur. This view of the cracking problem, while probably somewhat simplistic in some areas, gives a reasonable picture of the issues involved in cracking of bridge decks (Krauss and Rogalla, 1996).
Here is a simple example of the mechanics in action, from Krauss and Rogalla
(1996): …If the concrete has a free-shrinkage of 500 microstrain (με), but it is restrained and allowed to shorten only 250 με, the restraint is 50 percent. A concrete with a modulus of elasticity of 4 x 106 psi might have an effective modulus of only 2 x 106 psi, because of creep. The resultant stress would be the product of the strain (500 με) times the restraint (50 percent) times the effective modulus of elasticity (2 x 106 psi) for a resultant tensile stress of 500 psi. If the tensile strength of
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the concrete is greater than 500 psi, cracking will not occur. However, additional tensile stresses from thermal gradients or loading could crack such a concrete. Therefore, effects of shrinkage and temperature changes, effect concrete modulus, restraint conditions, tensile strength, and loading conditions must be considered. (Krauss and Rogalla, 1996)
Figure 3 shows the factors affecting cracking in bridge decks that are covered in this literature review.
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Cracking in
Bridge
Decks
Shrinkage Thermal Deflections Restraint Modulus of Creep of Geometry Tensile Corrosion Freeze/ Effects
Elasticity Concrete
Strength Thaw Plastic Heat of Live Loads Internal Rate of Gain Skew Rate of Chloride Air Content (Early Age) Hydration
Increase Permeability
Autogenous Temperature Formwork Reinforce-Depth of Ultimate Rebar Type
at Casting
ment Deck
Drying Cooling after Aggregate Cover
(Long Term) Batching
Carbonation Diurnal Fibers
Cycle
Annual External
Cycle
Solar Girders
Radiation
Heating
Compared Expansion
Joints
With Temp
at Casting
Coefficient of Thermal Expansion
Figure 3: Factors affecting cracking in bridge decks
30 2.1.3 Shrinkage
Shrinkage is thought to be one of the greatest causes of cracking in bridge decks (Krauss and Rogalla, 1996; Phillips et al., 1997). Restrained shrinkage alone can create tensile stresses sufficient to crack the deck. If the deck shrinks 500 microstrain, the deck can easily see tensile stresses exceeding 1000 psi, depending on the material properties and geometric constraints (Krauss and Rogalla, 1996).
There are four types of shrinkage of note. Plastic shrinkage occurs at early age, before the concrete has hardened. This type of shrinkage typically occurs because of poor curing conditions leading to evaporation of water and hence high capillary stresses. Autogenous shrinkage is based on the loss of water due to chemical consumption in the setting chemical reactions, and potentially the actual formation of the crystal structure. Drying shrinkage is the primary long-term shrinkage type, again based upon water loss. Carbonation shrinkage is a long-term shrinkage that occurs when there is a high CO2 concentration in the air around the concrete.
It must be noted that shrinkage as a whole is not well understood. The types of shrinkage can be isolated by using specific tests, but the actual mechanisms by which these shrinkage types proceed are open to argument.
2.1.3.1 Plastic (Early Age) Shrinkage
Plastic shrinkage occurs at early age. It is listed by Issa (1999) as the most important cause of bridge deck cracking. Plastic shrinkage depends on two primary factors: the
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rate at which surface water forms (bleeding) and the evaporation rate of the surface water (Wang et al., 2001). When the evaporation rate from the top surface of the concrete exceeds the bleed rate at which water rises from the concrete, the top surface dries out. At this point, the free water surface in the concrete drops within the concrete, yielding menisci between the particles. These menisci exert a tensile force due to surface tension on the particles, a suction of sorts. This and a low concrete strength due to top surface desiccation cause cracking (Mindess and Young, 1981; Cheng and Johnston, 1985; Holt, 2001; Brown et al., 2001). Since this type of cracking occurs because of forces near the surface of the concrete, the cracks are typically shallow in depth and originate from the top surface. These cracks, however, are sufficient to assist water and chloride penetration, and to provide stress concentration points for long-term shrinkage cracking. Plastic shrinkage does not require external restraint on the member to create stresses, as the majority of the member is not shrinking, and it is solely the surface that shrinks. Thus, the surface alone will crack. Typical cracks are no more than 2 or 3 feet long and are 2 to 3 inches deep (Xi et al., 2003, Krauss and Rogalla, 1996) and exhibit a typical “turkey track” configuration.
2.1.3.1.1 Curing conditions
Curing conditions are the overriding cause of plastic shrinkage cracking. It is the most common reason cited by transportation agencies for the transverse deck cracking (Krauss and Rogalla, 1996). Curing conditions are blamed by most departments of transportation for the early-age cracking problem. In many cases, the
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department of transportation’s specifications on bridge deck placement and curing may be ignored, greatly intensifying the problem.
There are several procedures that are important for limiting the plastic shrinkage cracking problem, all revolving around limiting evaporation from the fresh concrete. If possible, the evaporation rate should be measured or estimated, and the evaporation rate limited to 0.20 lb./ft2/hr for normal concrete and 0.10 lb./ft.2/hr. for concrete with a low water to cement ratio (Shing and Abu Hejleh, 1999). Evaporation counter measures are almost mandatory if the evaporation rate exceeds 0.20 lb./ft.2/hr, and cracking is possible even with an evaporation rate of only 0.10 lb./ft.2/hr (Cheng and Johnston, 1985). Nomographs are available to calculate the evaporation rate based on environmental conditions.
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Figure 4: Accumulation of early age and long term shrinkage, with various curing environments during the first day. Wind = 2 m/s (4.5mph), dry = 40% RH, wet = 100% RH. (Holt, 2001)
Figure 4 gives shows just how significant the curing conditions are in the shrinkage of concrete. Wind can greatly increase the shrinkage of concrete, and the level of wind shown (some 4.5 miles per hour) is often found on a jobsite. Dry conditions (like 40% relative humidity) are similarly commonly found, and proper precautions must be taken to prevent the drying shrinkage shown in the figure from occurring. Interestingly, it has been shown that there is no correlation between curing conditions in the first 24 hours and shrinkage at later times; they are essentially decoupled (Holt, 2001).
Moist curing for an extended period of time is highly recommended (Mindess and Young, 1981). Using a wet burlap system has long been considered the best method,
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but wind and heat can dry burlap rapidly, necessitating a method for keeping the burlap moist. The moist curing must start within a few minutes of the finishing to get the best results. Fogging during the time between strike-off and the application of the burlap helps reduce early-age plastic cracking as well, and is highly recommended (Xi et al., 2003; Shing and Abu-Hejleh, 1999; Cheng and Johnston, 1985).
Curing compounds can significantly reduce the number of small deck cracks, but this method is not as good as using wet burlap for several days. The film applied is difficult to make continuous, and the moisture from the wet curing aids the strength of the very top of the concrete.
2.1.3.1.2 Consolidation
It has been shown that inadequate consolidation contributes to early age cracking, as well as other issues. Typically, the department of transportation specifications are sufficient to prevent this problem, but are not always carried out in the field.
2.1.3.1.3 Finishing Procedures
Early finishing reduces the size and number of cracks. In addition, double-floated decks seem to have less cracking. In order to allow curing to commence earlier, it is recommended to saw cut the grooving rather than use rake tining of plastic concrete. Rake tining of plastic concrete damages the surface of the hardened concrete. Hand finishing should not be allowed except at the edge of the pavement (Krauss and Rogalla, 1996; Xi et al., 2003; Shing and Abu-Hejleh, 1999).
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2.1.3.1.4 Mix Design
The mix design of a concrete influences the plastic shrinkage. High water to cement ratios and high cement content increase plastic shrinkage (Aktan et al., 2003; Krauss and Rogalla, 1996). Interestingly, a high water to cement ratio would seem to lead to a higher bleed rate, which according to the accepted model of plastic shrinkage is a good thing. A lower water to cement ratio concrete would probably have its top surface dried out more readily. Early age cracking has become more prevalent as high performance concretes (with a low water to cement ratio) have become more common. Perhaps some further investigation of the relationship of water to cement ratio and plastic shrinkage is in order.
2.1.3.1.5 Admixtures
There are several admixtures that can impact the plastic shrinkage of concrete. Shrinkage reducing admixtures reduce the surface tension of the water in the capillary pores, thus reducing the stress from the pore water. This reduces the plastic shrinkage, but this mechanism also reduces air entraining, which may be problematic. Set retarders can actually increase plastic shrinkage simply by keeping the concrete plastic before setting for a longer period of time (Xi et al., 2003; ACI 212, 1989). Water reducing admixtures can help decrease the shrinkage as well by reducing the water to cement ratio.
2.1.3.1.6 Air temperature
The air temperature at batching directly influences the evaporation rate of the concrete, and thus the plastic shrinkage. It is typically recommended to batch when
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the air temperature is below 80° F (Xi et al., 2003; Krauss et al., 1995, Shing and Abu-Hejleh, 1999).
2.1.3.1.7 Wind
Several investigators and transportation departments consider wind to be the most significant factor affecting cracking (Krauss and Rogalla, 1996). Wind significantly increases evaporation, which is the main cause of plastic shrinkage cracking (Xi et al., 2003). Most sources recommend setting up temporary wind breaks during casting to limit evaporation until appropriate curing methods can be applied. Some curing procedures are adversely affected with wind, particularly any that have plastic sheeting placed, as the wind can blow under the plastic if the edges are improperly secured. If necessary, casting under a high wind condition should be avoided to reduce plastic shrinkage (Xi et al., 2003; Mindess and Young, 1981).
2.1.3.1.8 Humidity
Humidity decreases evaporation; to increase humidity around the concrete, foggers are often recommended. If the humidity in the air is very low, there can be high evaporation rates even without wind (Xi et al., 2003). More cracking has been observed for concrete cast during low humidities (Krauss and Rogalla, 1996).
2.1.3.1.9 Silica Fume Concrete
Silica fume increases the density of the concrete, decreasing porosity, and thereby also decreasing the bleed rate of the concrete. This inability of water to move within the mix increases the concrete’s susceptibility to plastic shrinkage and plastic
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shrinkage cracking. It has been shown that silica fume concrete is significantly more likely to crack if improper curing procedures are followed. However, studies have also shown that if appropriate curing procedures are adhered to, the silica fume does not increase plastic shrinkage cracking (Shing and Abu-Hejleh, 1999).
2.1.3.2 Autogenous Shrinkage
Autogenous shrinkage is defined as the macroscopic volume change occurring with no moisture transferred to the exterior surrounding environment, and thus is related to the actual chemical reactions of the concrete. Autogenous shrinkage occurs even when the concrete is completely submersed in water, thus having 100% humidity on the surface. It also occurs even when the surface is made completely air and water proof with some curing agent. Thus its mechanism is not related to surface tension of water at the surface, but rather to the surface tension in pores, a reduction in relative humidity as the pore water is chemically consumed, and the actual volume change from the reactants to the products (Xi et al., 2003; Holt, 2001; Brown et al., 2001; Lura, 2003). The higher performance concretes move the reaction more in favor of lower volume products, increasing the importance of the last mechanism mentioned.
Autogenous shrinkage is usually insignificant compared with plastic and drying shrinkage, but for high-strength concretes with low water-to-cement ratios, it has been shown that autogenous shrinkage becomes important. Most research indicates strength exceeding 6000 psi and water-to-cement ratios below 0.4 are most
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susceptible to autogenous shrinkage (Xi et al., 2003; Holt, 2001; Brown et al., 2001; Lura, 2003).
Autogenous shrinkage is a chemical shrinkage, but not all of the chemical shrinkage translates into autogenous shrinkage, which is an external measurement. Some of the chemical shrinkage ends up as voids in the concrete, as illustrated in Figure 5.
Figure 5: Reactions causing autogenous and chemical shrinkage (Holt, 2001 from Japan, 1999) C = unhydr ated cement, W = unhydr ated water , H y = h ydration pr oducts, and V = voids generated by hydration.
The first source of the chemical shrinkage is from volume reduction of the reaction products. This is dominant at very early age, when the concrete is still liquid. At this age, the chemical and autogenous shrinkage are equivalent. In addition, because the concrete is still liquid, the shrinkage does not result in stress, as the concrete is unrestrained and simply settles.
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After the skeleton of the concrete begins to be formed, there are several mechanisms in play. Figure 6 below illustrates the formation of empty pore volume due to chemical shrinkage, which results in a decrease of the radius of curvature of the menisci and in bulk shrinkage due to increased tensile stresses from the pore water. This is self desiccation shrinkage.
Figure 6: Sc hematic of a c ross-section of hydr ating c ement paste (Jense n and Hansen, 2000). Left: low degree of hydration. Right: high degree of hydration.
Self-desiccation is the most commonly cited mechanism, where the pore water is consumed by the hydration process. As the pores dry, the water menisci in the pores produce significant suction forces on the crystalline structure. Chemical shrinkage is still in play as the chemical reactions proceed and the products of the reaction form. These products are slightly less in volume than the reactants.
There is a third mechanism theorized that relates more to the concrete microstructure and gel formation. Surface tension of the gel particles has been proposed as the mechanism, but it could only be a small part of the autogenous deformation.
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The final mechanism proposed is disjoining pressure, where the adsorption of water to the gel particles is hindered. This occurs where the distance between the solid surfaces is less than two times the thickness of the free adsorbed water layer. The pressure is the result of van der Waals forces, double layer repulsion, and structural forces (Lura, 2003). This pressure is higher at higher relative humidity. When the relative humidity drops from water consumption, the disjoining pressure is reduced, causing shrinkage.
Autogenous shrinkage is hard to reduce without altering the actual water to cement ratio. If the autogenous shrinkage has to be reduced, it has been recommended that 25% of the coarse aggregate be replaced by a water-saturated lightweight aggregate (Xi et al., 2003). Holt (2001) agrees that the water to cement ratio is by far the most important factor in autogenous shrinkage, but lists three other factors that can influence it (shown in Figure 7). Holt was evaluating early age autogenous shrinkage for the most part, but noted three factors: bleed rate, chemical shrinkage, and time to hardening. A higher bleed rated decreases autogenous shrinkage, and earlier hardening does as well. Chemical shrinkage, the volume change when the hydration reaction progresses, directly influences autogenous shrinkage as well, but is generally not under the control of the engineer. Xi et al. (2003) lists these same factors as well.
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Figure 7: Direction of shift in early age autogenous shrinkage when influenced by other factors (Holt, 2001)
2.1.3.2.1 Mix design
Mix design is the factor with the largest influence on autogenous shrinkage. Autogenous shrinkage does not occur unless the water to cement ratio is below 0.42 (Holt, 2001). According to all sources, autogenous shrinkage increases as the water­to-
cement level decreases, particularly below about 0.4 (Shing and Abu-Hejleh, 1999).
2.1.3.2.2 Cement type
Type K cement has a different crystalline structure than standard Portland cements. This shrinkage-compensating cement actually expands as the concrete sets, compensating for other types of shrinkage. Since this occurs inside the concrete, it is an autogenous movement type.
The Ohio Turnpike Commission (OTC) has used type K concrete for many years, and has over 500 bridge decks with type K concrete. The New York Thruway Authority
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(NYTA) cast 47 decks in the early 1990s with this type of concrete. Linford and Reaveley (2004) reviewed the OTC and NYTA for their experiences with type K cement. The OTC has had good experience with type K decks, with most shrinkage cracking eliminated. They had to provide special treatment for the decks, including higher water to cement ratio, faster placement, faster implementation of curing, and continuous wet curing for 7 days. It must be noted that most of these are all well-known techniques for obtaining good shrinkage and cracking results, with or without the type K cement. NYTA had severe problems, and stopped using the cement. Overall, the benefits of type K are debated; some researchers show reduction in cracking, and others showed problems (Xi et al., 2003; Krauss and Rogalla, 1996).
2.1.3.3 Drying (Long Term) Shrinkage
Drying shrinkage is the most significant type of shrinkage in most concrete mixes, and has been called the most deleterious property of Portland cement composites (Zhang and Li, 2001). The mechanisms are similar to those of plastic shrinkage, but occur after the concrete has hardened. Drying shrinkage comes from the transfer of water from the concrete to the surrounding environment, thus increasing the surface tension in the pores. Eventually, the concrete will come to complete equilibrium with the surrounding environment. At that point the movement associated with moisture will simply follow the environmental conditions—if wet, then the concrete swells, if dry, it shrinks (Mindess and Young, 1981).
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There are three mechanisms described in the literature: capillary stress, disjoining pressure, and surface tension. Each of these mechanisms is dominant in a different range of relative humidity. The most important mechanism in field conditions is the capillary stress, which is dominant from 45%-90% humidity. The three mechanisms all appear to be reversible, but a large portion of the drying shrinkage is irreversible. The reason for the irreversibility is not well known; it is thought that the stresses from those three mechanisms cause the calcium silicate hydrate particles to realign to a “matrix stable” configuration. This realignment seems to only occur during the first drying period; after that, subsequent wetting and drying does not have a large impact on the irreversible part of drying shrinkage (Xi et al., 2003; Mindess and Young, 1981; Brown et al., 2001).
It is thought by most researchers that the ultimate shrinkage values are not the most important facet of the drying shrinkage issue. The actual rate of shrinkage is more important, as this compared with strength gain, creep and other time-dependent factors actually determines whether there will be cracking. If the shrinkage occurs quickly while the strength gain occurs slower, the concrete may crack early even though at the fully-developed values of both the concrete would have been strong enough to handle the load. In addition, if the shrinkage occurs quickly, creep is unable to relieve the stress. Xi et al. (2003) cite the following example: “For a concrete prism fully restrained at both ends, cracks may develop at a shrinkage strain of around 200~250 με if not accounting for the creep effect of concrete. Under high shrinkage rate, 200~250 με could easily occur at the age of 10 days under normal
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room temperature and 50% humidity. Therefore, proper measures must be taken to reduce not only the ultimate shrinkage strain but also the shrinkage rate.” It is generally perceived that reducing the shrinkage rate is more difficult than simply reducing ultimate shrinkage.
2.1.3.3.1 Curing methods
Curing of the concrete determines to a large extent the rate at which the drying shrinkage occurs (Krauss and Rogalla, 1996). If the concrete remains in a saturated condition, then drying shrinkage should be nearly eliminated for that period. Thus 7 day wet curing is very beneficial for letting the concrete gain strength before the shrinkage stresses cause cracking, and some even suggest 14 day. However, research by Holt (2001) shows that curing conditions for the first 24 hours do not affect shrinkage occurring at later ages. There seems to be some disagreement over how much curing conditions actually affect long-term behavior.
2.1.3.3.2 Mix Design
Mix design also has a significant impact on drying shrinkage. In particular, decreasing the water content decreases the drying shrinkage of the concrete. Interestingly, this is opposite to the results with autogenous shrinkage. The water to cement ratio has not been shown to have a conclusive effect on cracking, just on shrinkage. Decreasing the cement content decreases shrinkage, as the cement paste itself is the phase that causes the shrinkage. Essentially, high paste volume increases drying shrinkage. Many researchers have noted that high-slump concrete tends to increase cracking, which makes sense: high paste volume increases slump. Schmitt
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and Darwin (1999), for example, recommend that no more than 27% of the total volume of the concrete be cement and water (Schmitt and Darwin, 1999; Linford and Reaveley, 2004; Xi et al., 2003; Krauss and Rogalla, 1996; Cheng and Johnston, 1985).
Krauss and Rogalla (1996) list several other factors known to reduce drying shrinkage: maximizing the amount of aggregate (which reduces paste volume), using Type II cement, and using aggregate with low-shrinkage properties. A soft aggregate, such as sandstone, greatly increases the shrinkage of a concrete over a concrete using a hard aggregate (like dolomite); one researcher showed a 141 percent increase in that case. The absorption of the aggregate has been shown to reflect the drying shrinkage, but a quantitative relationship is not known (Babaei and Purvis, 1995; Cheng and Johnston, 1985). It is also known that cements from different sources can have widely different shrinkage characteristics; in some cases, one cement can have shrinkage over 100% higher than another (Babaei and Purvis, 1995).
2.1.3.3.3 Admixtures
Admixtures can modify the drying shrinkage. Shrinkage reducing admixtures reduce the surface tension in the pore water, reducing the driving force of the drying shrinkage, as well as the other types of shrinkage. Shrinkage reducing admixtures are very effective in reducing drying shrinkage (Xi et al., 2003). High range water reducers, retarders, and superplasticizers seem to have only a minor impact on drying shrinkage.
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2.1.3.4 Carbonation Shrinkage
Carbonation shrinkage occurs when the concrete is exposed to air with high concentrations of carbon dioxide and about 50% relative humidity for long periods of time. The concrete behaves as if it were exposed to drying conditions with a relative humidity far below the actual humidity (Brown et al., 2001). The conditions mentioned above occur most often in structures like parking garages, while bridges seldom have these conditions (Mindess and Young, 1981). Therefore, this type of shrinkage is outside the scope of this work, and will not be discussed further.
2.1.4 Thermal Effects
Thermal effects are as important to the cracking problem as shrinkage is, but are often overlooked since they are largely outside the control of the engineer. Nevertheless, the strain applied by temperature changes alone can easily be enough to cause cracking (Krauss and Rogalla, 1996; Aktan et al.,2003).
The thermal stress-free condition is locked in at the time and temperature of the concrete’s setting. From that time on, any temperature different than that experienced at the setting time will cause strain in the concrete. If this is restrained, then the strain is converted to stress. Differential stresses are created when the deck and the girders of a composite deck are expanding or contracting at different rates.
High early temperatures in the concrete can create early age cracks, as the thermal stresses act upon fresh concrete with low strength. Concretes that have high early
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strength usually also have a high heat of hydration, leading to more thermal cracking problems. To prevent excessive thermal gradients, the peak and placement temperatures of the concrete need to be limited, but how much is open to debate. There are numerous methods to reduce the heat related problems; they are discussed below.
2.1.4.1 Heat of Hydration
The heat of hydration for the concrete sets the baseline upon which all other thermal effects work. A high heat of hydration, combined with an early set time, will lead to an elevated stress-free temperature, which will greatly exacerbate the thermal movement problems. The problems depend also upon the geometry of the member; a large member will retain the heat generated by hydration longer, making a higher temperature when the concrete hardens more likely (Brown et al., 2001). If the concrete sets at, perhaps, 100° F, and the concrete eventually reaches 20° F at some later date, that thermal movement will add over 200 psi of tensile stress to the deck (Krauss and Rogalla, 1996). It is beneficial, therefore, to reduce the heat of hydration and to keep down the temperature at setting.
The heat of hydration is impacted by several factors. The most important is the cement type. A cement heavy in tricalcium silicate will have a much higher heat of hydration than one heavy in dicalcium silicate. Type III cement has the highest heat of hydration, both because of the high tricalcium silicate and tricalcium aluminate percentages, and because the clinker particles are ground to a smaller size, increasing
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their reactivity. Type I cement has a somewhat lower heat of hydration, and Type IV, specially designed to reduce the heat of hydration, has by far the lowest heat of hydration. Typically, the faster the cement gains strength, the higher the heat of hydration, because of the concentration of reactions in time—more reactions at the same time means more heat at that time. It is recommended that cements with a lower hydration heat be used where possible (Xi et al., 2003, Shing and Abu-Hejleh, 1999). In particular, Type II cement, which has slightly lower heat of hydration than Type I, is recommended for general purposes (Krauss and Rogalla, 1996; Shing and Abu-Hejleh, 1999; Babei and Purvis, 1995; Aktan et al., 2003).
For the concrete, however, there are other factors than simply the type of cement. The cement volume in the actual mix design also determines the concrete heat of hydration. Increasing the cement volume in the concrete increases the amount of heat generated by hydration.
Finally, some admixtures alter the heat of hydration. Retarders decrease the maximum heat of hydration by spreading out the hydration reactions in time, giving more time for the concrete to lose heat to the environment. In addition, fly ash has been successfully used to reduce cracking by reducing the strength gain and early concrete temperature (Krauss and Rogalla, 1996; Shing and Abu-Hejleh, 1999).
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2.1.4.2 Temperature at Casting
The actual temperature at the time of set determines the thermal behavior of the concrete from that time forward. Heat of hydration has a large influence on the setting temperature, but so do environmental conditions. The procedures used in the casting of the concrete can significantly modify the setting temperature as well.
If possible, the concrete should be cast at approximately the median temperature for the year; cracking is worse when the concrete is cast at either low or high temperatures (Krauss and Rogalla, 1996; Meyers, 1982; Cheng and Johnston, 1985). Obviously that is rarely possible, but it is possible to bring the temperature of the concrete close to that level. However, the temperature of the concrete at casting is rarely the temperature of the concrete at setting, because the concrete will quickly come to the temperature of the environment (Aktan et al., 2003). For this reason, it is unlikely that procedures such as cooling the mix with nitrogen actually have much impact on the setting temperature.
Agencies usually restrict batching temperature, both of the air and of the concrete itself. Concrete does not set properly at low temperatures; high temperatures cause problems with thermal movement. Air temperature at batching must be between 45° and 80° F (Rogalla et al., 2003). This is not practical in some regions of the country. Concrete mix temperatures must be above 50° F for the first 72 hours, and below 80° F (Xi et al., 2003, Shing and Abu-Hejleh, 1999; Krauss and Rogalla, 1996; PCA,
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1970). This is very difficult to attain if the air temperature is outside that envelope, because concrete quickly approaches the ambient temperature (Aktan et al., 2003).
2.1.4.2.1 Weather
The weather at the time of the concrete setting is important to the temperature of the concrete at setting. It is often recommended to batch late in the day during the summer months; this allows the setting of the concrete to take place late in the evening as the ambient temperature decreases. Night batching has been shown to significantly reduce deck cracking (Krauss and Rogalla, 1996; Purvis, 1989). In the winter, casting should take place so that the concrete will set during the warmest part of the day. These procedures will minimize the effect of the annual temperature cycle on the concrete. It is usually recommended not to batch when the temperature is above 80°.
2.1.4.2.2 Heat of hydration
The heat of hydration, as discussed above, will raise the concrete’s setting temperature. It is rarely feasible for the engineer to modify the mix to reduce the heat of hydration, as strength and shrinkage considerations dictate the mix proportions. Retarders are recommended to reduce the temperature gain from the heat of hydration (Xi et al., 2003).
2.1.4.2.3 Batching Temperature
During the winter and summer, the concrete is often warmed or cooled to meet department of transportation specifications on the temperature of the concrete at
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batching. In the winter, the aggregate is often heated through various means; in the summer, the water is chilled, ice is added, or the mix cooled with liquid nitrogen. Whether this does any good for the actual setting temperature is doubtful. Aktan et al. (2003) found that the concrete temperature at placement had little long term effect because the concrete quickly reached the ambient temperature.
2.1.4.3 Cooling After Batching
The first temperature change that the concrete will see is the actual cooling as the heat of hydration is released. This can very often cause cracking, because the concrete is still weak, but the matrix itself has already formed. The restraint provided by underlying beams and the forms themselves is sufficient to translate the strain into stress. Cracking from this source is usually formed above the uppermost transverse bars and is full depth (Xi et al., 2003).
Krauss and Rogalla (1996) give an example of the potential stress generated by the
cooling of a deck that was 50° F above the temperature of the restraining girders: A 28° C (50° F) temperature change in the deck relative to the girders can cause stresses greater than 1.38 MPa (200 psi) when the concrete has an early effective modulus of elasticity of only 3.5 GPa (0.5 x 106 psi), and greater than 6.89 MPa (1000 psi) when the early effective modulus is 17.2 GPa (2.5 x 106 psi).
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2.1.4.4 Diurnal Cycle
A concrete bridge deck’s temperature will mirror to an extent the ambient conditions of the surrounding environment. The heat of a bridge deck will vary as much as 50° Fahrenheit during the course of a day. This type of thermal movement is too short-term to be alleviated by creep, and thus must be taken by the concrete itself (if restrained). This is the primary source of thermal stress, since the change is non­uniform
on the structure; this non-uniformity is covered in the solar radiation section (Xi et al., 2003; Krauss and Rogalla, 1996).
Krauss and Rogalla (1996) give examples of the levels of thermal stress from the diurnal cycle that can be reached, from analytical analysis of the system. The assumption in these examples is of a linear temperature gradient in the bridge. With a 50° F temperature change, the tensile stresses can reach 1350 psi on simply-supported steel girders, and 1480 psi on simply-supported concrete girders. Over the interior supports of a continuous span bridge, the tensile stress could reach 2000 psi on concrete girders. Those numbers were calculated theoretically from the mechanics of the system; in reality, the concrete would probably fail long before those stresses were reached.
2.1.4.5 Annual Cycle
The annual temperature cycle also brings significant temperature fluctuations to the bridge deck. During a year, the high temperature during a day may go from 0° to 100° Fahrenheit. This type of fluctuation is less problematic, because it is uniform
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across the structure. Thus, the girders and deck will see precisely the same changes. If the deck and girders have the same coefficient of thermal expansion, little stress will be seen. However, when the girders are steel, the total temperature change is the source of the stress, rather than the differential change across the structure (Xi et al., 2003). When combined with the diurnal cycle, the annual cycle brings a temperature range of some 120°, and that is just the air temperature in the surrounding environment. This range is what has to be handled when the deck and girders are not the same material. The concrete itself is also likely to get hotter from radiation—but since that heating is non-uniform and non-linear, it is considered in the next section.
The annual temperature cycle is another of the factors that the engineer has no control over, but it is useful to consider it. Krauss and Rogalla developed equations to calculate the stress developed in a concrete bridge deck with various conditions. Obviously, the worst condition would have the concrete and the girders see different temperatures; if they differ by 50° after the stress-free temperature for the combination is when they are the same temperature, the tensile stress in the concrete can approach 1000 psi, far beyond the tensile capacity of the concrete. However, in most cases the stresses from the annual cycle are limited, since the concrete and steel have at least similar coefficients of thermal expansion (Krauss and Rogalla, 1996).
2.1.4.6 Solar Radiation Heat
This is one of the worst temperature impacts on the bridge deck. The sun heats the top surface, while the bottom surface remains relatively cool, particularly if over a
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large body of water. This yields very significant differential strains, causing curvature and stress in the deck; the free deck will try to curve convex upward. If the solar radiation heating is sustained for a full day, eventually the deck will increase in temperature significantly, while the underlying girders remain relatively cool. This can again put significant stress into the concrete (Krauss and Rogalla, 1996). Figure 8 (Figure 1 from Krauss and Rogalla) illustrates these different types of thermal movements. When these strains are translated to stresses (Figure 9), the stresses can be very large. Figure 9 is also from Krauss and Rogalla, and gives results of a typical calculation. They undertook a large number of similar calculations to determine the maximum stresses that could be seen by the girders and deck.
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Figure 8: Strain effects of various temperature changes (Krauss and Rogalla, 1996)
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Figure 9: Example deck and steel girder stresses for various temperature changes (Krauss and Rogalla, 1996).
2.1.4.7 Compared with temperature at casting
The strain in the concrete depends on the difference between the concrete temperature and that at which the concrete set. The only thing the engineer can control to any degree is the batch temperature, which should be somewhere between the extremes to try to reduce the maximum strains seen.
2.1.4.8 Coefficient of Thermal Expansion
The coefficient of thermal expansion determines how large the strains are with the variation in temperature. This is essentially beyond the control of the engineer.
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However, the differing thermal coefficients of concrete and steel may explain why it has been seen that steel girder bridges are somewhat more prone to cracking than concrete girder bridges. At the time of setting, the stress-free temperature is set, with the concrete usually at a slightly higher temperature than the girders. Then, as the annual and diurnal temperature cycles occur, the concrete deck and steel girders move at different rates, causing stresses to occur in the system.
The coefficient of thermal expansion of concrete is from 4 to 7 με/°F, while that for steel is 7 με/°F (Xi et al., 2003; Shing and Abu-Hejleh, 1999; Mindess and Young, 1981). Concrete with a higher coefficient of thermal expansion is theoretically desirable on a steel girder bridge, in order to match the movement of the girders, but this also would increase the thermal stresses from other sources (like temperature gradients in the deck from radiation), reducing any benefit (Xi et al., 2003).
2.1.4.8.1 Aggregate
The aggregate used has a large impact on the coefficient of thermal expansion. However, it is rarely feasible for aggregates to be chosen based on the thermal expansion coefficient. The final coefficient of thermal expansion is a combination of the coefficients of the cement matrix and that of the aggregate; the paste coefficient is usually 2 to 3 times higher than that of the aggregate (Mindess and Young, 1981; Krauss and Rogalla, 1996; Xi et al., 2003).
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2.1.5 Deflections
This is the third and least important source of strain in the concrete. It, like much of the temperature strain, is of short duration, so the strain cannot be relieved by creep.
2.1.5.1 Live Loads
These obviously produce both stress and strain in the concrete, both after curing and potentially during the curing process if the concrete feels vibrations induced by traffic. These loads are added to those from shrinkage and thermal factors, but it is typically considered that these loads are not significant in the cracking problem. This is because the stresses induced are usually much lower than those from other sources, and they are usually compressive for the deck as well. In addition, these are the loads that the decks are actually designed to carry. Traffic-induced vibrations during curing have not been found to be detrimental (Krauss and Rogalla, 1996).
2.1.5.2 Formwork
The formwork potentially can induce strain, as it is holding the concrete in a certain position during casting. When removed, the structure settles into its dead-load deflected shape, inducing tensile strain in the concrete. There has been some research done on types of formwork, with inconclusive results on whether there is a correlation between formwork type and cracking of the deck. Some advocate stay-in-place forms, while others say they increase the cracking (Krauss and Rogalla, 1996; Cheng and Johnston, 1985). Nothing conclusive has been determined.
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The other type of strain associated with formwork comes from deflection of the formwork while the concrete is plastic. Cracking may occur over the supports of continuous deck bridges in this condition; this situation can be eliminated by using appropriate pour sequences to eliminate formwork deflection inducing tensile stresses in those locations (Krauss and Rogalla, 1996). It should be noted that this type of job sequencing may cause cold joints and construction difficulties.
2.1.6 Restraint
Without restraint, the strain would simply cause movement of the concrete. However, bridge decks are highly restrained systems, both internally and externally. When restraint is present, the strain is converted to stress according to the modulus of elasticity of the concrete (assuming linear elastic behavior). There are two classes of restraints: internal and external. The internal restraint on a bridge deck comes from the reinforcement in the deck, from the aggregate in the deck, and from any fibers in the deck. The external restraint comes from the girders and from any end restraints; the expansion joints are planned to reduce external restraint. However, if the girders and deck are composite, as is often the case, nearly all of the external restraint comes from the girders anyway (Krauss and Rogalla, 1996, Brown et al., 2001).
2.1.6.1 Internal
There are several sources of internal restraint to the concrete matrix. The reinforcing steel is chosen to carry load, but it also is a restraint to the concrete. When the concrete shrinks, the reinforcement does not, thus inducing tensile stress in the concrete and compressive stress in the reinforcement.
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2.1.6.1.1 Reinforcement
The rebar imbedded in the concrete provides a significant degree of longitudinal restraint, and to some extent lateral as well. Since the loads are most significant longitudinally, where they can accrue along the length of the bridge, this is a problem for the bridge deck. Embedded reinforcement, to a lesser extent than girders, restrains the deck against shrinkage and thermal movement, as the coefficient of thermal expansion of the reinforcement is likely different from that of the deck. Of course, the engineer cannot remove the reinforcement from the deck, but there are a few factors that are under the engineer’s control.
2.1.6.1.1.1 Epoxy coated
Epoxy coated rebar behaves differently in its interaction with concrete than does standard rebar. It has been shown that bridges with epoxy-coated rebar behave worse than those with standard black rebar. There is an increasing likelihood for cracking shown, and the epoxy-coated bars develop considerably less bond stress. The cracks tend to be larger with the epoxy-coated rebar (Krauss and Rogalla, 1996; Meyers, 1982). The epoxy rebar helps chloride-ion protection in the laboratory under ideal conditions, but in practice there has not been any benefit found. In addition, the epoxy sometimes delaminates from the steel, causing a failure zone to develop at the bonding surface (Linford and Reaveley, 2004).
2.1.6.1.1.2 Rebar location
Some researchers felt like the rebar location, particularly how much cover was present, had an impact on the cracking. It has been shown that cracking tends to
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occur over the transverse reinforcing steel. It is possible that this occurs because of insufficient cover at those locations. As the concrete settles in the plastic phase, a zone of weakness tends to develop over the rebar, which fractures first under the stresses leading to cracking (Aktan et al., 2003; Issa, 1999; Linford and Reaveley, 2004; Babaei, 2005).
2.1.6.1.2 Aggregate
It has been shown that the aggregate types have a significant impact on all facets of concrete behavior. Aggregate provides a large measure of the concrete’s internal restraint. However, it is rarely feasible to choose aggregate types based upon the measure of internal restraint provided. Aulia (2002) demonstrated that the type of aggregate had a significant impact on the properties of the concrete.
Clean, low shrinkage aggregate is important in getting a high quality concrete. It is well known that the type of aggregate has a significant impact on shrinkage of the concrete, and on the time to crack as well (Krauss and Rogalla, 1996).
Larger aggregate is recommended in a number of sources, in order to minimize the paste volume without sacrificing workability (Xi et al., 2003, PCA, 1970; Shing and Abu-Hejleh, 1999). In addition to minimizing paste volume, the larger aggregates tend to bear directly on one another, so shrinking paste cannot move them. This tends to channel the stress into microcracks within the cement paste, rather than shrinkage. As long as these microcracks to not turn into larger cracks, the effect is considered
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beneficial. It is commonly recommended to achieve the highest possible aggregate volume in the mix, as less paste decreases shrinkage and thermal problems (Xi et al., 2003). “In general, concrete mixes with good quality, clean, low shrinkage aggregate with high aggregate to paste ratio have been observed to perform better (Saadeghvaziri and Hadidi, 2002).”
2.1.6.1.3 Fibers
Fibers provide internal restraint as well, particularly against movement before curing. Steel fibers will continue to provide restraint after curing, as their high modulus of elasticity will continue to take load. Polymer fibers stop providing restraint once the concrete’s modulus of elasticity becomes higher than the fibers’. There is some question whether early restraint is beneficial or detrimental to the concrete. If the concrete is still in the plastic stage, there would not be any stress captured in the matrix, so it likely doesn’t hurt to have this early restraint.
2.1.6.2 External
The external restraint on bridge decks is also significant. The girders are the primary source of the restraint. The best-case scenario is if the girders and deck are cast monolithically; then the shrinkage stresses are equal, and the thermal effects are minimized as well (Krauss and Rogalla, 1996). Most bridges, however, have the deck cast independently from the girders, and are composite systems.
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2.1.6.2.1 Girders
The girders are the portion of the bridge in contact with the deck, and thus their composition and design can influence the behavior of the bridge deck. As the deck contacts the girders all along the length of the deck, and shear systems such as shear studs are used, longitudinal movement of the deck relative to the girders is prevented. Girders restrain the deck movement whenever they do not have temperature or shrinkage strains identical to the deck. Because steel girders do not experience any long term drying shrinkage, they tend to exert greater restraint on the deck than concrete girders. Since only a portion of the deck is restrained, there are induced stresses from the eccentric restraint present as well (Krauss and Rogalla, 1996).
When large girders are used, they can restrain approximately 60% of the uniform free strain at the upper surface of the deck; smaller girders can restrain 35 to 45% of the free strain at the upper surface (Krauss and Rogalla, 1996). Of course, there are many other variables as well.
If the deck has a linear free strain rather than a uniform free strain, the deck tries to curve to alleviate this. This type of movement is restrained at a much higher percentage, from 75 to 95% (Krauss and Rogalla, 1996).
2.1.6.2.1.1 Concrete vs. steel
Due to the fact the steel has a different coefficient of thermal expansion than concrete, the degree of restraint placed by differential movement depends on the
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material of the beams. In addition, the steel has a higher modulus of elasticity, leading to a higher degree of restraint on any free strain in the deck. Finally, the steel girders do not shrink like the concrete deck; the concrete deck strain is completely restrained by the girders. This, combined with the thermal difficulties, explains why cracking is more common on steel girder structures (Xi et al., 2003, Aktan et al., 2003; Krauss and Rogalla, 1996; Meyers, 1982; Cheng and Johnston, 1985; Linford and Reaveley, 2004).
2.1.6.2.1.2 Continuous vs. Simply-Supported
It is thought that continuous-span structures are more susceptible to cracking than simple-span structures (Krauss and Rogalla, 1996; Meyers, 1982; Aktan et al., 2003; Linford and Reaveley, 2004). This is likely due to the negative moment regions over supports and to the longer stretches of deck without any expansion joints. The negative moment regions induce tension in the deck over the support, which a deck already in tension due to shrinkage and potentially thermal effects is ill-prepared to withstand (Cheng and Johnston, 1985; Perfetti et al., 1985).
2.1.6.2.1.3 Girder size and spacing
Research indicates that the size and spacing of the girders effect cracking, but as these are designed based on other issues, they cannot be altered simply to protect the bridge deck. Restraint is increased with larger girders, and with more girders; higher restraint increases the likelihood of cracking (Shing and Abu-Hejleh, 1999).
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2.1.6.2.1.4 Composite deck/girder systems
Composite decks and girders are the norm in bridge design, as they greatly improve the efficiency of the load-carrying system. Most of the discussion of restraints thus far has assumed that the deck and girders act compositely. However, these systems are the source of much of the restraint upon the system. If the deck and girders did not act compositely, the deck would be free to move with shrinkage and thermal strains to a much greater extent (Krauss and Rogalla, 1996). It is not a coincidence that the cracking problem became much more pronounced as the use of a composite deck/girder system became common.
However, it would be premature to advocate the return to noncomposite systems. Further research into the relative merits of the systems is in order, however, particularly in light of the high cost of repairing and replacing cracked decks.
2.1.6.3 Expansion joints
The design and placement of expansion joints can affect how well movements are taken up by the bridge, but they cannot alleviate restraint placed on the deck by the simple presence of the girders.
2.1.7 Modulus of elasticity
The modulus of elasticity of concrete is poorly understood, in that the modulus of concrete changes both over time and with loading. According to Krauss and Rogalla (1996), the modulus of elasticity affects the stresses in the concrete more than any other property. The modulus of elasticity determines the conversion ratio of strain to
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stress in the concrete (Xi et al., 2003). As the strain is the given for both shrinkage and thermal movements, a lower modulus of elasticity will decrease the stress in the concrete. However, a lower modulus of elasticity comes from a concrete with a lower binder ratio, and thus usually a lower strength as well.
A concrete’s modulus of elasticity approximately mirrors the concrete’s strength (Xi et al., 2003). It is unclear if there is any net benefit from reducing the binder ratio, since the strength is usually reduced. Of course, the external loads apply a given stress to the system, so a lower modulus of elasticity will increase deflections--except that the effect will simply be a reduction of the load taken by the deck and an increase of the load taken by the girders (whose modulus of elasticity is a constant).
To reduce the modulus of elasticity without reducing the strength, the primary approach is to use aggregates with a low modulus of elasticity (Xi et al., 2003; Krauss and Rogalla, 1996). Aulia (2002) also found that the modulus of elasticity was largely dependent on the aggregate used, and demonstrated that the relationship held true in fiber-reinforced concrete as well. Whether choosing aggregate to give a low modulus of elasticity is practicable depends on the location where the concrete is batched.
2.1.7.1 Modulus gain
There is some research done of the modulus gain curves. These curves essentially mirror the strength-gain curves of the concrete. In order to get better crack
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performance, Xi et al. (2003) recommend that a concrete with low early strength and modulus of elasticity be used. However, the cracking performance depends on the relationship of tensile strength to the modulus of elasticity, and that relationship is very hard to determine, so attempting to avoid cracking by using a low modulus concrete may not succeed.
2.1.8 Creep of Concrete
Creep of concrete is one factor beneficial to the engineer. Creep occurs with when the concrete is under load for long periods of time. Over time, the concrete slowly moves away for the load, deforming according to the load. Essentially, concrete tries to alleviate stress by a restructuring of the matrix. There are two types of creep: basic creep, which occurs without moisture movement to or from the environment, and drying creep, which is the additional creep caused by drying. The differences between these types of creep, and the fact that there is no distinct separation between instantaneous strain and time-dependent strain, make quantifying creep difficult (Linford and Reaveley, 2004). Research has been done on how great a benefit can be expected from creep and what influences its behavior. Krauss and Rogalla (1996) list creep as one of the major factors effecting bridge deck cracking. Creep occurs in the cement paste; aggregates do not creep. However, lower modulus aggregates encourage creep, possibly by increasing the localized stress in the cement paste (Xi et al., 2003). The nature of creep itself is not well understood; the mechanism seems to be related to the response of calcium silicate hydrate to stress—calcium silicate hydrate has multiple phases it may switch between (Mindess and Young, 1981).
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It has been shown that the tensile creep can relax shrinkage stresses by up to 50%, doubling the strain failure capacity. Both the magnitude and time history of the shrinkage stress influence the time of cracking. Altoubat and Lange (2002) showed that the tensile creep caused their sample mixes to crack at twice the expected failure time based on shrinkage analysis for high performance concrete, and three times the expected failure time for the standard mixtures. Interestingly, they found that the actual evolution of the stress greatly altered the creep behavior. Concrete in a restrained shrinkage test that was sealed for three days and then unsealed actually cracked earlier than unsealed concrete. This, they believe, comes from the higher modulus of elasticity of the sealed concrete, and the exposure shock acceleration of the shrinkage. In addition, they showed that periodic wetting increased the creep of the concrete.
The creep of concrete typically mirrors the compression strength of the concrete. The creep rate (the concrete’s rate of relaxation) decreases at a faster rate than the modulus of elasticity and tensile strength increases. This allows the stress into the concrete to catch up to the tensile strength over time (Figure 10). Note the tensile strength curve is flatter than the stress gain curve (Brown et al., 2001).
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Figure 10: Time depe ndence of res trained shrinkage, creep, and tensile strength (Brown et al., 2001 after Mehta, 1993)
2.1.8.1 Mix Design
There has been research done on exactly what types of mixes creep more or less. In particular, concrete with higher water content creeps more (Krauss and Rogalla, 1996). Since higher water content also increases shrinkage, it is unclear whether this addition of water is actually beneficial. Increasing cement paste volumes increase the creep potential (Xi et al., 2003).
As the compressive strength of a concrete increases, creep decreases and tensile strength increases. However, the creep decreases at a much greater rate than the increase of the tensile strength. This helps to explain why higher strength concretes usually have worse crack performance than normal strength concretes (Xi et al., 2003).
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2.1.8.2 Curing Conditions
Curing conditions significantly modify the creep behavior of concrete. Drying creep dominates basic creep (creep not depending on air drying) on bridge decks, which are usually drying from both sides. “Drying creep is typically 2 to 3 times basic creep when the air relative humidity is 70 to 50 percent, respectively (Krauss and Rogalla, 1996).”
2.1.8.3 Admixtures
Addition of retarders can increase the creep at early age, which can relieve more of the early age shrinkage and thermal issues. Slower curing mixes have higher creep (Krauss and Rogalla, 1996).
2.1.8.4 Plastic Settlement
Plastic settlement of concrete occurs while the concrete is still fresh. As water rises to the surface, the concrete subsides. If there is insufficient cover, cracking will occur over the top reinforcement as the concrete subsides on either side. Babaei (2005) considers this one of four primary causes of bridge deck cracking.
2.1.9 Geometry
The geometry of the design can influence bridge deck cracking, as it can influence stress concentrations and differential movements. This is a very complex subject, and thus difficult to make generalizations about, but a few things are known about how geometry influences bridge deck cracking.
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2.1.9.1 Skew
Some respondents in the survey indicated that they thought skew increased cracking, probably because of stress concentrations. Krauss and Rogalla (1996) believe that skew does not significantly affect transverse cracking, but that it does cause slightly higher stresses near the corners. One researcher (Purvis, 1989) found bridges with a skew over 30 ° were more susceptible to transverse cracking.
2.1.9.2 Depth of Deck
The depth of deck influences the differential movements associated with solar radiation heating of the top surface and can also influence other temperature effects, as the inner core will retain heat longer. However, for actual concentration of stresses, the depth of deck has a minimal impact. Though research is lacking, the information that there is indicates that thinner decks lead to more cracking (Xi et al., 2003; French et al., 1999).
2.1.9.3 Cover
It is believed that the concrete cover does have an impact on deck cracking, but there is not a consensus on what that impact is. Shallow cover increases the likelihood of settlement cracking (Krauss and Rogalla, 1996; Cheng and Johnston, 1985). However, if the cover gets too deep, over about 3 inches, the steel reinforcement is less effective at distributing tensile stresses (Krauss and Rogalla, 1996). Some researchers found worse cracking with cover over 3 inches while others found no correlation. Top cover between 1.5 and 3 inches is recommended (Xi et al., 2003; Krauss and Rogalla, 1996; PCA, 1970).
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2.1.10 Tensile Strength
The tensile strength of the concrete determines if the concrete will actually crack. Unfortunately, concrete is very weak in tension and the actual tensile strength is poorly understood, as it changes with time. The tensile strength of concrete is often estimated as 10% of the concrete’s compressive strength (ACI Committee 318, 2002). The actual tensile strength is subject to considerable fluctuation from sample to sample, because the tensile strength is very sensitive to anything acting as a stress concentrator or crack initiator. Once the concrete starts cracking in tension, it fails almost instantly.
The concrete cracks when the stress is higher than the tensile strength at that time. If the stresses develop faster than the strength, the concrete will crack at early age. Figure 11 shows the tensile strength curve—when the stress reaches the tensile strength, the concrete will crack.
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Figure 11: Time depende nce of res trained shrink age, stress relaxation (cree p), and tensile strength (Brown et al., 2001 after Mehta, 1993)
To further complicate matters, some evidence shows that the concrete cracks below its tensile strength. Table 1 shows some results obtained by Altoubat and Lange (2002) showing that the concrete was cracking at a restrained shrinkage stress below that of the direct tensile strength. Likely this would be due to the likelihood of flaws in larger samples causing cracking to propagate at a lower stress level.
Table 1: Restrained shrinkage stresses and age at cracking (Altoubat and Lange, 2002)
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There are two important factors: the rate of increase and the ultimate strength. If the tensile strength of the concrete rises at a fast enough rate, it can outpace stresses induced by shrinkage at early age, preventing cracking at early age. Long term, the ultimate tensile strength needs to be high enough to resist all stresses that come upon it. There are several factors that can increase tensile strength.
2.1.10.1 Fibers
Fibers can greatly help tensile strength at early age. However, polymer fibers have a modulus of elasticity lower than that of hardened concrete, and thus do not help long term. It has been shown that steel fibers increase ultimate tensile strength. The fibers are potentially very beneficial in increasing the rate of increase of the tensile strength, thus avoiding early age cracking (Kao, 2005).
2.1.10.2 Mix Design
A stronger concrete will have a higher tensile strength. Thus, lower water to cement ratios, higher cement contents, and other factors that are known to increase concrete compression strength will also increase the tensile strength. Unfortunately, these factors usually also increase shrinkage and thermal problems, so if trying to limit cracking, often it is not beneficial to increase the concrete’s strength.
2.1.11 Corrosion
Corrosion is often a long term cracking problem. Much of the corrosion problems come from having existing cracks that allow ingress of water and salts. These cracks accelerate the corrosion problem, which increases the cracking problem.
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2.1.11.1 Chloride Ion Penetration
Different types of concrete corrode at different rates, depending on the permeability of the concrete and the degree of passivation. Silica fume has been added to increase the density of the concrete, but many researchers indicate that silica fume increases sensitivity to curing procedures. If the concrete is cured properly, cracking can be avoided for the most part (Shing and Abu-Hejleh, 1999). Silica fume has a high heat of hydration, is sticky, and is expensive; these issues tend to negate the benefits in ion penetration (Xi et al., 2003).
2.1.11.2 Rebar Type
Epoxy-coated rebar has not been shown to reduce the corrosion problem in the field. In the lab, it performs well, but that is under ideal conditions. After handling in the field, the epoxy has shown both delamination and scratching. Epoxy-coated rebar recovered from failed structures often show delamination and corrosion problems. Epoxy rebar tends to localize the corrosion, increasing the rate of corrosion at those places. It has been shown that cracks tend to larger in bridge decks with epoxy-coated rebar (Krauss and Rogalla, 1996; Meyers, 1982; Linford and Reaveley, 2004).
Stainless-steel rebar does not corrode, but it is very expensive and has only been used by one Department of Transportation, Oregon’s. Stainless-clad rebar seems to be a viable alternative, as it costs only some 50% more than standard rebar and shows significant resistance to corrosion.
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2.1.12 Department of Transportation Opinions
Many papers have been published that include results of surveys on the causes of bridge deck cracking. In addition, many Departments of Transportation commissioned researchers to evaluate what the causes of bridge deck cracking were in their state. These causes may be mechanical, procedural, or a number of other things. A brief review of the surveys and opinions of the reports are presented here.
Krauss and Rogalla (1996) surveyed 52 agencies in the United States and Canada. Most of the respondents indicated that they considered early transverse cracking a problem; nearly all report extensive cracking on bridge decks. The agencies were requested to indicate what they thought to be the causes of bridge deck cracking. Table 2 gives the results of that question; the number in parentheses is the number of responses giving that cause.
Twenty agencies (out of fifty-two) consider improper curing to be a cause of cracking. Wind, thermal effects, and air temperature were each listed by seven agencies. These cannot be remediated easily, but correcting the curing problems should be a high priority. The most common materials problem cited was concrete shrinkage, with drying shrinkage specifically singled out. A few of the agencies also considered deflection design to be a reason for cracking in bridge decks.
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Table 2: Causes of bridge deck cracking, agency survey (Krauss and Rogalla, 1996)
Construction
Materials
Design
improper curing (20) wind (7) thermal effects (7) air temperature (7) relative humidity (4) vibration (2) placement conditions/weather (2)
concrete shrinkage (17) [5 cited drying shrinkage specifically] concrete mix design (7) plastic shrinkage (3) excessive cement (3) concrete temperature (3) use of retarders (2)
deflections (7) excessive cover (3) placement sequence (2)
In addition, Krauss and Rogalla ranked the causes of bridge deck cracking they evaluated from their own research and many other sources. Table 3 gives those findings. These findings are very similar to those discussed earlier throughout the analysis of the mechanical causes of cracking.
78 Table 3: Factors affecting bridge deck cracking (Krauss and Rogalla, 1996)
79 The Kansas Department of Transportation recommended that a silica fume overlay be used to decrease permeability. In addition, wet cure specifications were recommended. They used wet burlap for 7 days, and it cut cracking by 50%. Finally, they liked polymer overlays, but recommend a heavy grit blast (#6 or #7) (Xi et al., 2003).
According to the Utah Department of Transportation report (Linford and Reaveley, 2004), there are a number of factors causing cracking. Restrained shrinkage is listed as the most common cause. Issa (1999) suggests ten causes, listed in order of descending importance:
1.
Inadequate concrete curing procedures which result in high evaporation rates and thus a high magnitude of shrinkage, especially in early age concrete.
2.
The use of high slump concrete.
3.
High water-to-cement ratios due to inadequate mixture proportions and retempering of concrete.
4.
Insufficient top reinforcement cover.
5.
Inadequate vibration of the concrete.
6.
Deficient reinforcing details of the joint between a new and old deck.
7.
Sequence of deck section placement.
8.
Vibration and loads from machinery.
9.
The weight of concrete forms.
10.
Deflection of formwork.
80
The Utah Department of Transportation analysis of their bridges found that composite deck attachment to girders, bents, diaphragms, and abutments exacerbated the cracking problem, as it increased the restraint of the deck. Steel girders, as opposed to concrete girders, greatly increased the cracking problem; this is probably because of the differences in thermal expansion coefficients or the difference in thermal mass. Large concrete placements also increase cracking.
The Michigan Department of Transportation report (Aktan et al., 2003) included analysis of a database of inspections. They had several conclusions:
•
More cracks were observed on the continuous bridges than the simple span bridges.
•
Bridges with PCI (Precast Prestressed Concrete Institute) girders showed less longitudinal crack density than other girder types.
•
More transverse and diagonal cracks were observed on bridges with adjacent box girders than other girder types.
•
Map cracking was only observed on bridges with steel girders.
Xi et al. (2003) conducted an analysis of Colorado bridges for the Colorado Department of Transportation, and developed a list of recommendations as well. They recommended:
•
Type II cement or Type I cement with increased fly ash.
•
Cement content below 470 lb/yd3 if possible.
•
Water to cement ratio around 0.4.
81
•
At least 20% Type F fly ash.
•
Maximum 5% silica fume.
•
May use ground granulated blast furnace slag to improve durability.
•
Specify strength at 1, 3, 7, 28, and 56 days.
•
Consider using permeability, drying shrinkage, and crack resistance tests as acceptance tests.
•
Largest aggregate size possible and well graded aggregate to minimize cement
paste volume. In addition, they recommended a number of things regarding design factors, primarily aimed at minimizing restraint. For construction practice, it is recommended that the air temperature be between 45° and 80° F for batching, and generally to reduce evaporation however possible. They recommended 7 day continuous moist curing. (Shing and Abu-Hejleh,1999; Xi et al., 2003)
The Michigan report (Aktan et al., 2003) gives the responses of thirty-one Departments of Transportation in regards to the causes of bridge deck cracking. Each respondent was asked to give the three top causes of bridge deck cracking in their jurisdiction. Figure 12 gives the responses.
82
What are the top three causes of early age bridge deck cracking in your jurisdiction?
results of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curing
Substandard Thermal Restraint Mix Design Structural Epoxy Rebar Construction Other Curing Stress System Practice
Causes
Figure 12: Frequency of top three causes of early-age bridge deck cracking (Aktan et al., 2003)
Research in the U. K. has indicated that their early age cracking problem is primarily due to restraint of early thermal movement, rather than restraint of shrinkage as previously thought (The Highways Agency, 1989). The researchers note that cracking has become more prevalent in recent years, as higher strength concretes have been implemented; higher strength concretes usually also produce more heat in the curing period. Thermal movement would be of little consequence if the member was unrestrained, but bridge decks are highly restrained by the beams on which they rest. In plain concrete, thermal cracking tends to yield a few wide cracks; minimal temperature reinforcement leads to more and smaller cracks.
Babaei (2005) reduced all the causes of bridge deck cracking to four central points: settlement of plastic concrete, thermal shrinkage of curing concrete, drying shrinkage
83 of hardened concrete, and flexure. The causes for each of these mechanical processes are then identified and possible methods for reduction given.
Plastic settlement occurs as the concrete bleeds. Often, voids develop under transverse reinforcement bars where rising water collects, and a crack develops above, due to the restraint upon settlement at that location. Several factors promote this condition: shallow cover, a higher slump mix, and large reinforcement size. Babaei constructed a table showing the probability of cracking based on these conditions (Table 4).
Table 4: Probability of Plastic Shrinkage Cracking (Babaei, 2005)
Probability of Cracking (percent)
2 in. slump
3 in. slump
4 in. slump
Bar Size
#4
#5
#6
#4
#5
#6
#4
#5
#6
¾ in. cover
80%
88%
93%
92%
99%
100%
100%
100%
100%
1 in. cover
60%
71%
78%
73%
83%
90%
85%
95%
100%
1.5 in. cover
19%
35%
46%
31%
48%
59%
44%
61%
72%
2 in. cover
0%
2%
14%
5%
13%
26%
5%
25%
39%
Thermal shrinkage during curing is another major type of problem. The concrete cures at high temperature from the heat generated by hydration. It then cools, but is restrained from shrinking by the beams, causing stresses in the deck. Cracking thus occurs as the deck cools.
Babaei states that the difference between the deck and beam temperature contributes strain at the rate of about 5.5 microstrain/degree F. Creep cannot compensate,
84 because the stresses are fully realized within a few days. A temperature differential of about 40 degrees F is enough to produce cracking without other factors; other factors such as drying shrinkage contribute to cracking with less temperature differential. It is best, therefore, to keep the differential to 22 degrees F or less. To do this, less cement, Type II cement, or retarders are recommended. In addition, precautions should be taken in cold weather.
Drying shrinkage cracking is the third type of problem addressed by Babaei. This occurs over long periods of time, on the order of a year. Assuming that creep is 50% of shrinkage, 400 microstrains of drying shrinkage would be needed to crack the concrete. An 8 to 9 inch thick deck can shrink up to about 550 microstrains, depending on the mix. The deck shrinkage is about 2.5x less than that of standard ASTM shrinkage prisms. Therefore, a reasonable parameter for maximum long term specimen shrinkage (assuming deck/beam thermal differential of 22F) would be about 700 microstrains. For 28 day shrinkage, that number would be about 400 microstrains.
There are several factors affecting drying shrinkage cracking mechanically. Aggregate mineralogy is one; porous, “soft” aggregate concrete can have shrinkage twice that of concrete with hard, non-porous aggregates. The type and source of cement also impacts shrinkage; it is best to use cement from a proven source, and type II if possible. If admixtures are used, it is important to test the mix beforehand
85
in case unforeseen interactions occur. Finally, minimizing the water in the concrete is key.
The final primary cause of cracking in the opinion of Babaei is from flexure, particularly from unshored construction in continuous bridge decks. To minimize early cracking from this source, it is best to place the deck concrete in midspan first. This minimizes the movement in the area over the support after that section is placed. (Babaei, 2005)
It appears, then, that the causes of cracking are many and varied. Design, construction, and materials issues are all considered contributors. Many point to curing problems as a primary cause of cracking. A large proportion point to shrinkage problems associated with the mix design. A number of design issues seem to be neglected as well, though often designs are non-negotiable in most aspects. It seems that thermal problems are largely ignored. The number of causes is large, and a number of actions not common in construction could help reduce cracking.
2.1.13 Application in the Field
The Michigan Department of Transportation report (Aktan et al., 2003) gave an interesting commentary on the code and adherence to the code by the contractors. Construction monitoring of projects was conducted to see whether contractors adhered to the MDOT Standard Specifications for Construction. There were a number of areas that did not meet the standards:
86
•
Freefall of the concrete was often more than 6 inches.
•
Vibratory compaction was often not done within 15 minutes of placing, as concrete delivery delays sometimes exceeded 30 minutes.
•
Vibrators were not used in a pattern, but rather randomly. Vibrators seemed to be used to move concrete into place.
•
Curing was applied for 7 days, but proper precautions were not taken to ensure it was a wet cure operation (which was required).
•
Curing compound was applied very late, rather than immediately after bleed water had left. Sometimes the entire deck was placed before curing compound was applied.
•
Far more than the maximum of 10 feet of textured concrete were left exposed without curing compound.
•
Burlap was not applied until the next day, and then not properly wetted. It was supposed to be placed as soon as the concrete surface could support it, not more than two hours after pouring.
•
Proper procedures for keeping burlap wet were not followed; no soaker hoses were used.
•
The expansion joint boundaries are problematic. Excess concrete overflows, loses its plasticity, and is scraped off and thrown in with the deck concrete near the joint. Concrete that falls off the joints should not be placed back on the deck. (Aktan et al., 2003)
Thus, it appears that even if the departments of transportation have appropriate specifications in place for curing and other construction issues, these specifications
87
are not always followed. In design, deck cracking problems are generally ignored as a design parameter. Concrete mix designs are usually created to maximize strength and other parameters such as freeze-thaw resistance, but shrinkage and crack resistance are generally relegated to secondary consideration.
2.1.14 Summary/Conclusion
The United States has a vast bridge deck cracking problem, which has grown in recent years with the increasing use of high strength concrete and the commonplace usage of composite girder/deck designs. There are several key improvements that can help improve the cracking problem.
This literature review has discussed the mechanics of bridge deck cracking. Many causes of bridge deck cracking were identified, but not all are under the control of the engineer. Figure 13 attempts to illustrate the areas where the engineer has good control of the causes of cracking. Many aspects of the bridge design are controlled by the geometry and loads, so the engineer has only minimal control. Some areas, like the thermal movement, are environmental conditions. There are several key areas where the engineer has good control: plastic and drying shrinkage of the concrete deck, the restraint in the deck provided by fibers, and the rebar type used. With these, and making good choices where only moderate control is possible, cracking can be controlled.
88
Cracking in
Bridge
Decks
Factors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint,

CREATION OF AN ODOT SPECIFICATION FOR
PATCHING OR OVERLAY OF BRIDGE DECKS
FINAL REPORT - FHWA-OK-08-09
ODOT SPR ITEM NUMBER 2184
By
Chris C. Ramseyer
Assistant Professor
Daniel S. Myers
Research Assistant
Civil Engineering and Environmental Science
University of Oklahoma
Norman, Oklahoma
March 2009 TECHNICAL REPORT DOCUMENTATION PAGE
1. REPORT NO. FHWA-OK-08-09
2. GOVERNMENT ACCESSION NO.
3. RECIPIENT=S CATALOG NO.
4. TITLE AND SUBTITLE CREATION of an ODOT Specification for Patching or Overlay of Bridge Decks
5. REPORT DATE September 2009
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S) Chris C. Ramseyer and Daniel S. Myers
8. PERFORMING ORGANIZATION REPORT
9. PERFORMING ORGANIZATION NAME AND ADDRESS University of Oklahoma 202 w. Boyd, room 334 Norman, Oklahoma 73019
10. WORK UNIT NO.
11. CONTRACT OR GRANT NO. ODOT Item Number 2184
12. SPONSORING AGENCY NAME AND ADDRESS Oklahoma Department of Transportation Planning and Research Division 200 N.E. 21st Street, Room 3A7 Oklahoma City, OK 73105
13. TYPE OF REPORT AND PERIOD COVERED Final Report From October 2004 To October 2008
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16 . ABSTRACT Bridge deck cracking is a huge problem in the United States, and various agencies have sponsored research endeavoring to determine the underlying problems. A number of causes have been identified, including thermal movement, plastic shrinkage, and early age settlement, as well as a number of other issues. Polymer fibers are a possible solution to many of the causes of bridge deck cracking: they have been shown to help early age properties like shrinkage and movement, and as a bonus, fibers improve post-cracking behavior. More understanding of the benefits and uses of polymer fibers in concrete is needed. This study researched the properties of four polymer fibers; two of the fibers were macrofibers, and two were microfibers. Each fiber was tested at several dosage rates to identify optimum dosage levels. Early age shrinkage, long term shrinkage, compressive strength, and tensile strength were investigated. Macrofibers and microfibers were found to have different impacts on concrete behavior, with different optimal dosage rates. Microfibers greatly dried out the concrete mixture, hindering workability. However, the microfibers substantially reduced plastic shrinkage and improved concrete strength at early age. Macrofibers, while not hindering workability, did not provide benefits as great as the microfibers to the concrete strength.
17. KEY WORDS Very Early Strength Concrete, Accelerator, Rapid Strength Concrete, Patching, Concrete Repair
18. DISTRIBUTION STATEMENT No restrictions. This publication is available from the Planning & Research Division, Oklahoma DOT.
19. SECURITY CLASSIF. (OF THIS REPORT) Unclassified
20. SECURITY CLASSIF. (OF THIS PAGE) Unclassified
21. NO. OF PAGES 98
22. PRICE N/A
ii SI (METRIC) CONVERSION FACTORS
Approximate Conversions to SI Units
Approximate Conversions from SI Units
Symbol When you know Multiply by To Find Symbol
Symbol When you know Multiply by To Find Symbol
LENGTH in inches 25.40 millimeters mm ft feet 0.3048 meters m yd yards 0.9144 meters m mi miles 1.609 kilometers km AREA in² square inches 645.2 square millimeters mm ft² square feet 0.0929 square meters m² yd² square yards 0.8361 square meters m² ac acres 0.4047 hectares ha mi² square miles 2.590 square kilometers km² VOLUME fl oz fluid ounces 29.57 milliliters mL gal gallons 3.785 liters L ft³ cubic feet 0.0283 cubic meters m³ yd³ cubic yards 0.7645 cubic meters m³
LENGTH mm millimeters 0.0394 inches in m meters 3.281 feet ft m meters 1.094 yards yd km kilometers 0.6214 miles mi AREA mm² square millimeters 0.00155 square inches in² m² square meters 10.764 square feet ft² m² square meters 1.196 square yards yd² ha hectares 2.471 acres ac km² square kilometers 0.3861 square miles mi² VOLUME mL milliliters 0.0338 fluid ounces fl oz L liters 0.2642 gallons gal m³ cubic meters 35.315 cubic feet ft³ m³ cubic meters 1.308 cubic yards yd³
MASS oz ounces 28.35 grams g lb pounds 0.4536 kilograms kg T short tons 0.907 megagrams Mg (2000 lb) TEMPERATURE (exact) ºF degrees (ºF-32)/1.8 degrees ºC Fahrenheit Celsius FORCE and PRESSURE or STRESS lbf poundforce 4.448 Newtons N lbf/in² poundforce 6.895 kilopascals kPa per square inch
MASS g grams 0.0353 ounces oz kg kilograms 2.205 pounds lb Mg megagrams 1.1023 short tons T (2000 lb) TEMPERATURE (exact) ºC degrees 9/5+32 degrees ºF Celsius Fahrenheit FORCE and PRESSURE or STRESS N Newtons 0.2248 poundforce lbf kPa kilopascals 0.1450 poundforce lbf/in² per square inch
iii The contents of this report reflect the views of the author(s) who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the views of the Oklahoma Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. While trade names may be used in this report, it is not intended as an endorsement of any machine, contractor, process, or product. CREATION OF AN ODOT SPECIFICATION FOR PATCHING
OR OVERLAY OF BRIDGE DECKS
FINAL REPORT
By
DANIEL S. MYERS
Research Assistant
Under the Supervision of
Chris C. Ramseyer, Ph.D., P.E.
Assistant Professor
Civil Engineering and Environmental Science
University of Oklahoma
202 W. Boyd, room 334
Norman, Oklahoma 73019
March 2009
v Table of Contents
Acknowledgements
..........................................................................................
iv
List of Tables
...................................................................................................
xii
List of Figures
................................................................................................
xiii
Abstract
........................................................................................................
xviii
Chapter 1
: Introduction
............................................................................
20
Chapter 2
: Literature Review
..................................................................
21
2.1
Bridge Deck Cracking
...............................................................................
21
2.1.1
Scope of the Problem
.....................................................................
22
2.1.2
Mechanics of Cracking
..................................................................
24
2.1.3
Shrinkage
.....................................................................................
31
2.1.3.1
Plastic (Early Age) Shrinkage
...............................................
31
2.1.3.2
Autogenous Shrinkage
..........................................................
38
2.1.3.3
Drying (Long Term) Shrinkage
............................................
43
2.1.3.4
Carbonation Shrinkage
..........................................................
47
2.1.4
Thermal Effects
............................................................................
47
2.1.4.1
Heat of Hydration
.................................................................
48
2.1.4.2
Temperature at Casting
.........................................................
50
2.1.4.3
Cooling After Batching
.........................................................
52
2.1.4.4
Diurnal Cycle
........................................................................
53
2.1.4.5
Annual Cycle
........................................................................
53
vi 2.1.4.6
Solar Radiation Heat
.............................................................
54
2.1.4.7
Compared with temperature at casting
.................................
57
2.1.4.8
Coefficient of Thermal Expansion
........................................
57
2.1.5
Deflections
...................................................................................
59
2.1.5.1
Live Loads
............................................................................
59
2.1.5.2
Formwork
..............................................................................
59
2.1.6
Restraint
.......................................................................................
60
2.1.6.1
Internal
..................................................................................
60
2.1.6.2
External
.................................................................................
63
2.1.6.3
Expansion joints
....................................................................
66
2.1.7
Modulus of elasticity
....................................................................
66
2.1.7.1
Modulus gain
........................................................................
67
2.1.8
Creep of Concrete
........................................................................
68
2.1.8.1
Mix Design
............................................................................
70
2.1.8.2
Curing Conditions
.................................................................
71
2.1.8.3
Admixtures
............................................................................
71
2.1.8.4
Plastic Settlement
..................................................................
71
2.1.9
Geometry
......................................................................................
71
2.1.9.1
Skew
......................................................................................
72
2.1.9.2
Depth of Deck
.......................................................................
72
2.1.9.3
Cover
.....................................................................................
72
2.1.10
Tensile Strength
...........................................................................
73
2.1.10.1
Fibers
.....................................................................................
75
vii
2.1.10.2
Mix Design
............................................................................
75
2.1.11
Corrosion
......................................................................................
75
2.1.11.1
Chloride Ion Penetration
.......................................................
76
2.1.11.2
Rebar Type
............................................................................
76
2.1.12
Department of Transportation Opinions
......................................
77
2.1.13
Application in the Field
...............................................................
86
2.1.14
Summary/Conclusion
...................................................................
88
2.2
Fiber-Reinforced Concrete
.....................................................................
92
2.2.1
Fiber Material Properties
.............................................................
92
2.2.2
Workability
..................................................................................
94
2.2.3
Early Age Shrinkage
....................................................................
95
2.2.4
Long Term Shrinkage
..................................................................
96
2.2.5
Compression Strength
..................................................................
97
2.2.6
Tensile Strength
...........................................................................
99
2.2.7
Flexure
.........................................................................................
100
2.2.8
Modulus of Elasticity
...................................................................
101
2.2.9
Failure Types
...............................................................................
101
2.2.10
Fibers as Crack Inhibitors
............................................................
102
2.2.10.1
Crack Width and Time to Cracking
......................................
102
2.2.10.2
Impact Resistance
.................................................................
103
2.2.11
Fiber-Reinforced Concrete: Conclusion
......................................
103
2.3
Literature Review: Conclusion
...............................................................
104
Chapter 3
: Research Scope
.....................................................................
106
viii
3.1
Tests
...........................................................................................................
107
3.1.1
Fresh Concrete Tests
....................................................................
107
3.1.2
Compression Strength
..................................................................
109
3.1.3
Tensile Strength
...........................................................................
110
3.1.4
Unrestrained Shrinkage
................................................................
111
3.1.5
Unrestrained Shrinkage from Time Zero
.....................................
112
3.2
Matrix
........................................................................................................
116
3.3
Fibers
.........................................................................................................
118
3.3.1
Stealth
..........................................................................................
118
3.3.2
Grace Microfiber
........................................................................
119
3.3.3
Strux 90/40
.................................................................................
120
3.3.4
High Performance Polymer (HPP)
.............................................
121
3.4
Base Mix
..................................................................................................
122
3.5
Typical Batching Procedure
..................................................................
123
3.5.1
Pre-batching preparation
............................................................
123
3.5.2
Batching Procedure
....................................................................
125
Chapter 4
: Results
.................................................................................
127
4.1
Fresh Concrete Tests and Conditions
..................................................
128
4.2
Compression Tests
.................................................................................
131
4.3
Splitting Tensile Tests
............................................................................
134
4.4
Unrestrained Shrinkage
........................................................................
136
4.5
Unrestrained Shrinkage from Time Zero
............................................
138
4.6
Fiber-Reinforced Concrete: Summary of Results
..............................
140
ix
4.6.1
Stealth Summary
........................................................................
140
4.6.2
Grace Microfiber Summary
.......................................................
141
4.6.3
Strux 90/40 Summary
................................................................
141
4.6.4
HPP Summary
............................................................................
142
Chapter 5
: Discussion
...........................................................................
143
5.1
Workability
.............................................................................................
143
5.1.1
Slump
.........................................................................................
144
5.1.2
Finishing
....................................................................................
146
5.2
Fresh Concrete Characteristics
............................................................
147
5.3
Shrinkage
................................................................................................
149
5.3.1
The Unrestrained Shrinkage from Time Zero Test
....................
150
5.3.2
Shrinkage from Time Zero
.........................................................
157
5.3.2.1
Shrinkage from Time Zero: Stealth
....................................
157
5.3.2.2
Shrinkage from Time Zero: Grace Microfiber
....................
158
5.3.2.3
Shrinkage from Time Zero: Strux 90/40
.............................
159
5.3.2.4
Shrinkage from Time Zero: HPP
........................................
160
5.3.3
ASTM Unrestrained Shrinkage
..................................................
161
5.4
Plain Concrete Control Mixes
...............................................................
163
5.4.1
Plain Concrete: Fresh Concrete Properties
................................
164
5.4.2
Plain Concrete: Shrinkage from Time Zero
...............................
165
5.4.3
Plain Concrete: ASTM Unrestrained Shrinkage
........................
166
5.4.4
Plain Concrete: Compression Strength
......................................
167
5.4.5
Plain Concrete: Splitting Tensile Strength
.................................
167
x
5.5
Fiber Evaluation
.....................................................................................
169
5.5.1
General Survey of Fibers
...........................................................
169
5.5.1.1
Unrestrained Shrinkage from Time Zero
............................
169
5.5.1.2
ASTM Unrestrained Shrinkage
...........................................
170
5.5.1.3
Compression Strength
.........................................................
171
5.5.1.4
Splitting Tensile Strength
...................................................
174
5.5.2
Stealth Optimum Dosage
...........................................................
176
5.5.3
Grace Microfiber Optimum Dosage
..........................................
179
5.5.4
Strux 90/40 Optimum Dosage
...................................................
181
5.5.5
HPP Optimum Dosage
...............................................................
185
5.6
Microfiber and Macrofiber comparison
..............................................
188
5.7
Impact of Fibers: Summary
..................................................................
189
Chapter 6
: Conclusions
.........................................................................
191
References
......................................................................................................
193
xi
List of Tables
Table 1
: Restrained shrinkage stresses and age at cracking (Altoubat and Lange, 2002)
.....................................................................................................................
74
Table 2: Causes of bridge deck cracking, agency survey (Krauss and Rogalla, 1996)
...............................................................................................................................
78
Table 3
: Factors affecting bridge deck cracking (Krauss and Rogalla, 1996)
............
79
Table 4
: Probability of Plastic Shrinkage Cracking (Babaei, 2005)
...........................
84
Table 5
: Primary matrix
..............................................................................................
117
Table 6
: Primary matrix testing regimen
....................................................................
117
Table 7
: Base mix
.....................................................................................................
123
Table 8
: Primary matrix batches
...............................................................................
127
Table 9
: Primary matrix batching conditions
............................................................
128
Table 10
: Primary matrix fresh concrete properties
................................................
130
Table 11
: Primary matrix compression test results
...................................................
133
Table 12
: Primary matrix splitting tensile strength
...................................................
136
Table 13
: ASTM unrestrained shrinkage test results (normalized at 1 day)
.............
137
Table 14: Unrestrained shrinkage from time zero tests results (normalized at time 0)
.............................................................................................................................
139
Table 15
: Plain concrete fresh concrete properties and batch conditions
.................
164
Table 16
: Macrofiber and microfiber comparison
....................................................
189
Table 17
: General impact of fibers
...........................................................................
190
xii
List of Figures
Figure 1
: Causes of bridge deck cracking (Brown, et al., 2001)
..................................
24
Figure 2
: Time dependence of restrained shrinkage and creep (Brown et al., 2001
after Mehta, 1993)
...................................................................................................
27
Figure 3
: Factors affecting cracking in bridge decks
..................................................
30
Figure 4
: Accumulation of early age and long term shrinkage, with various curing
environments during the first day. Wind = 2 m/s (4.5mph), dry = 40% RH, wet =
100% RH. (Holt, 2001)
.........................................................................................
34
Figure 5
: Reactions causing autogenous and chemical shrinkage (Holt, 2001 from
Japan, 1999)
..........................................................................................................
39
Figure 6
: Schematic of a cross-section of hydrating cement paste (Jensen and Hansen,
2000). Left: low degree of hydration. Right: high degree of hydration
.............
40
Figure 7
: Direction of shift in early age autogenous shrinkage when influenced by
other factors (Holt, 2001)
......................................................................................
42
Figure 8
: Strain effects of various temperature changes (Krauss and Rogalla, 1996) 56
Figure 9
: Example deck and steel girder stresses for various temperature changes
(Krauss and Rogalla, 1996)
...................................................................................
57
Figure 10
: Time dependence of restrained shrinkage, creep, and tensile strength
(Brown et al., 2001 after Mehta, 1993)
.................................................................
70
Figure 11
: Time dependence of restrained shrinkage, stress relaxation (creep), and
tensile strength (Brown et al., 2001 after Mehta, 1993)
.......................................
74
Figure 12
: Frequency of top three causes of early-age bridge deck cracking (Aktan et
al., 2003)
...............................................................................................................
83
xiii
Figure 13
: Factors affecting cracking in bridge decks: level of engineer control
......
89
Figure 14
: Effect of paste volume fraction on workability of steel fiber-reinforced
mortars with 30 mm fibers (Johnston, after Pfeiffer and Soukatchoff, 1994)
......
94
Figure 15
: Air content pressurized air pot apparatus
..................................................
108
Figure 16
: Slump test apparatus
..................................................................................
108
Figure 17
: Compression test with Forney compression testing machine
...................
110
Figure 18
: Splitting tensile test
...................................................................................
111
Figure 19
: Unrestrained shrinkage test
.......................................................................
112
Figure 20
: Unrestrained shrinkage from time zero test in progress
............................
113
Figure 21
: Time zero molds prepared for filling
........................................................
114
Figure 23
: Stealth Microfibers
..................................................................................
119
Figure 24
: Grace Microfibers
....................................................................................
120
Figure 22
: Strux 90/40 Fibers
...................................................................................
121
Figure 25
: HPP (High Performance Polymer) fibers
................................................
122
Figure 26
: Coarse aggregate pile
..............................................................................
124
Figure 27
: Batching area
...........................................................................................
125
Figure 28
: Environmental chamber and samples: A – 4x8” cylinders, B – unrestrained
shrinkage from time zero samples, C – ASTM unrestrained shrinkage samples, D
–
restrained ring tests (not used in this research)
................................................
126
Figure 29
: Plain concrete compression failure: brittle
..............................................
131
Figure 30
: Fiber-reinforced concrete compression failure: ductile (Strux 90/40 10lb
dosage)
................................................................................................................
132
Figure 31
: Fiber-reinforced concrete splitting tensile failure: ductile
......................
135
xiv
Figure 32
: Slump versus fiber dosage
.......................................................................
144
Figure 33
: Concrete mixture with high dosage of HPP fibers
..................................
145
Figure 34
: Concrete finish on HPP high dosage mix at time of casting
...................
146
Figure 35
: Concrete finish on Strux 90/40 high dosage mix after unmolding
..........
147
Figure 36
: Unit weight versus fiber dosage
..............................................................
148
Figure 37
: Air content versus fiber dosage
...............................................................
149
Figure 38
: Time Zero Mold Comparisons to 24 hours (Strux 90/40 1lb)
.................
150
Figure 39
: Comparison of Time Zero Molds from 4 hours (Strux 90/40 1lb)
..........
151
Figure 40
: Average time zero versus average ASTM unrestrained shrinkage
.........
152
Figure 41
: Time Zero versus ASTM Unrestrained Shrinkage
..................................
153
Figure 42
: Time Zero versus ASTM Unrestrained Shrinkage (Strux 90/40 high
dosage rates)
........................................................................................................
154
Figure 43: Time Zero versus ASTM Unrestrained Shrinkage (HPP high dosage rates)
.............................................................................................................................
154
Figure 44
: Accumulation of early age and long term shrinkage, with various curing
environments during the first day. Wind = 2 m/s (4.5mph), dry = 40% RH, wet = 100% RH. (Holt, 2001)
.......................................................................................
156
Figure 45
: Time zero shrinkage results: Stealth
.......................................................
158
Figure 46
: Time zero shrinkage results: Grace Microfiber
.......................................
159
Figure 47
: Time zero shrinkage results: Strux 90/40
................................................
160
Figure 48
: Time Zero shrinkage results: HPP
...........................................................
161
Figure 49
: Unrestrained shrinkage at 28 days (bars show data range)
.....................
162
Figure 50
: Unrestrained shrinkage curves: HPP
.......................................................
163
xv
Figure 51
: Plain concrete shrinkage from time zero
.................................................
165
Figure 52
: Plain concrete ASTM unrestrained shrinkage (bars show data range)
...
166
Figure 53
: Plain concrete compression strength (bars show data range)
..................
167
Figure 54
: Plain concrete splitting tensile strength (error bars show data range)
.....
168
Figure 55
: Unrestrained shrinkage from time zero: 24 hour readings
......................
170
Figure 56
: ASTM unrestrained shrinkage at 28 days (bars show data range)
..........
171
Figure 57
: Compression strength at 24 hours (bars show data range)
......................
172
Figure 58
: Compression strength at 28 days (bars show data range)
.......................
173
Figure 59
: Splitting tensile strength at 24 hours (bars show data range)
..................
174
Figure 60
: Splitting tensile strength at 28 days (bars show data range)
...................
176
Figure 61
: Stealth slump versus fiber dosage and shrinkage at 24 hours
.................
177
Figure 62
: Stealth strength at 24 hours
.....................................................................
177
Figure 63
: Stealth strength at 28 days
.......................................................................
178
Figure 64
: Grace Microfiber slump versus fiber dosage and shrinkage at 24 hours 179
Figure 65
: Grace Microfiber 24 hour strengths
........................................................
180
Figure 66
: Grace Microfiber 28 day strengths
..........................................................
181
Figure 67
: Strux 90/40 slumps
..................................................................................
182
Figure 68
: Strux 90/40 plastic shrinkage
..................................................................
183
Figure 69
: Strux 90/40 24 hour strength
...................................................................
184
Figure 70
: Strux 90/40 28 day strength
....................................................................
185
Figure 71
: HPP slumps
.............................................................................................
186
Figure 72
: HPP plastic shrinkage results
..................................................................
186
Figure 73
: HPP 24 hour strengths
.............................................................................
187
xvi
Figure 74: HPP 28 day strength................................................................................ 188
xvii
Abstract
Bridge deck cracking is a huge problem in the United States, and various agencies have sponsored research endeavoring to determine the underlying problems. A number of causes have been identified, including thermal movement, plastic shrinkage, and early age settlement, as well as a number of other issues. Polymer fibers are a possible solution to many of the causes of bridge deck cracking: they have been shown to help early age properties like shrinkage and movement, and as a bonus, fibers improve post-cracking behavior. More understanding of the benefits and uses of polymer fibers in concrete is needed.
This study researched the properties of four polymer fibers; two of the fibers were macrofibers, and two were microfibers. Each fiber was tested at several dosage rates to identify optimum dosage levels. Early age shrinkage, long term shrinkage, compressive strength, and tensile strength were investigated.
Macrofibers and microfibers were found to have different impacts on concrete behavior, with different optimal dosage rates. Microfibers greatly dried out the concrete mixture, hindering workability. However, the microfibers substantially reduced plastic shrinkage and improved concrete strength at early age. Macrofibers, while not hindering workability, did not provide benefits as great as the microfibers to the concrete strength.
xviii
In general, several key results were identified, and it is suggested that many of these impacts can be explained by considering that the polymer fibers have a modulus of elasticity well below that of the hardened concrete matrix. Fibers were found to greatly reduce early age shrinkage, with the effect increasing with increasing dosage levels. Long term shrinkage is not affected by the addition of polymer fibers. Early age concrete strength is improved with the addition of fibers, but long term strength is sometimes reduced with high dosages of fibers. It is noted that these characteristics of polymer fibers indicate that they will be very useful in combating the bridge deck cracking problem.
xix
Chapter 1: Introduction
Bridge decks have many problems with cracking. More than 100,000 bridge decks, nearly half of the bridges in the United States, showed transverse cracking at early age (Krauss and Rogalla, 1996). Early age cracking is the most common deck distress reported by the State Highway Agencies. In all , 97% of state Departments of Transportation indicated that they have problems with early age cracking (Aktan et al., 2003).
Numerous studies have been performed on these problems, and several of the primary causes have been isolated. These include thermal movement, early age shrinkage, and early age settlement (Krauss and Rogalla, 1996; Babaei, 2005). These causes may all be counteracted by the addition of polymer fibers. Polymer fibers have been shown to be beneficial to the early age properties of concrete, as well as to crack mitigation (Kao, 2005).
Research presented here analyzes a number of fibers and dosage rates for their strength and shrinkage properties. Four types of fibers are tested; each one is tested at three to five different dosage rates. The results indicate that long term strength is not strongly impacted by polymer fiber addition, but early age shrinkage is greatly decreased and early age strength is increased.
20
Chapter 2: Literature Review
There has been considerable research work done on both ends of the field: bridge deck cracking and fiber reinforcement. General reviews of the bridge deck cracking problem have been conducted by the National Cooperative Highway Research Program (NCHRP) and several Departments of Transportation (DOT’s). These reviews analyze the problems statistically, and provide a summary of many variables important to the problem. Fiber reinforcement has typically been regarded as a simple crack reducer, but there is research investigating many aspects of its impact on material properties. Fibers impact the bridge deck cracking problem on several fronts, not simply by bridging cracks. A review of research done on both bridge deck cracking and fiber-reinforced concrete is presented here.
2.1 Bridge Deck Cracking
Bridge deck cracking is a problem throughout the United States, as several surveys indicate. A number of state departments of transportation, including Michigan, Texas, Oregon, Utah, New Jersey, Minnesota and Colorado have launched studies on the problem (Brooks, 2000; Brown et al., 2001; Xi et al., 2003; Aktan et al., 2003; Linford and Reaveley, 2004), and in 1996 NCHRP conducted a major project entitled “Transverse Cracking in Newly Constructed Bridge Decks”. This project, undertaken by Krauss and Rogalla, was a comprehensive analysis of the cracking problem at that point, and set out the problems in great detail. Since then a number of projects have conducted research according to the recommendations of that report. The departments of transportation performed similar analyses, researching the problem
21
statistically through surveys, and then identifying the primary causes of cracking. Applicable laboratory research and extensive field studies on new bridges were done to test various methods of mitigating the problem.
An interesting aspect of the present cracking problem is that it has increased as the strength of the concretes used has increased. This may indicate that something about the newer high-performance concretes encourages cracking, unless some other variable such as workmanship or curing is becoming worse during the same period of time. This literature review will investigate why that may be, and what to do about it (Xi et al., 2003).
2.1.1 Scope of the Problem
A large proportion of the bridges in the United States crack at early age. Aktan et al. (2003) found that early age cracking is the single most prevalent deck distress reported by the State Highway Agencies. More than 100,000 bridge decks in the United States showed transverse cracking at early age, according to Krauss and Rogalla (1996); this is nearly half of the bridges. Their survey included 52 DOT’s in the United States and Canada. Sixty-two percent of these agencies considered transverse cracking a problem; fifteen percent believed that all of their bridges suffered from transverse cracking. The respondents stated that, on average, forty-two percent of bridge decks cracked in the first week.
22
In the report for the Utah Department of Transportation (Linford and Reaveley, 2004) a database of 71 newly-constructed bridges in the I-15 reconstruction project was created. The bridges were constructed between April 1998 and March 2001. The bridges were each ranked with a Cracking Severity Index Number (CSIN). Cracking was found on 70 of the 71 bridges. Diagonal cracking was found on 87% of the bridges, primarily near abutments or interior bents. Transverse cracking was found on 67% of the bridges; according to the report, this, they postulate, was caused by concrete shrinkage. Only 11% have visible longitudinal cracks.
The report for the Colorado Department of Transportation (Xi et al., 2003) analyzed 72 structures built between 1993 and 2000. These were inspected in 2002. At that time, 82% of the bridges had deck cracking. In addition, the report declared that the Nevada Department of Transportation stated that 75% of all new bridges have a significant cracking problem. The Kansas Department of Transportation indicated that their cracking problems have been mostly resolved; they attribute their success to the implementation of a wet burlap 7 day curing procedure, which cut deck cracking by 50%.
Michigan conducted a survey of the state Departments of Transportation in 2002 (Aktan et al., 2003). Thirty-one states responded. Of these, 97% indicated that they had an early age cracking problem in reinforced concrete bridge decks. Nearly all of those first observed bridge deck cracking within the first year, and most within the first few months. Seventy-eight percent of the respondents stated that transverse
23
cracking was the most prevalent, with 16% citing longitudinal cracks, and 6% diagonal.
2.1.2 Mechanics of Cracking
Krauss and Rogalla (1996) carefully considered the mechanics of the cracking problem in their report on transverse cracking in bridge decks. Concrete bridge decks develop cracks when the tensile stress in the concrete exceeds the tensile strength of the concrete at that time. The tensile stresses come from concrete shrinkage, temperature changes in the concrete, and sometimes from self-weight or traffic loads. The stresses develop in the bridge decks because the girders restrain the natural thermal and shrinkage movement of the deck, thus translating the strain into stress.
Brown et al. (2001) endeavored to further isolate the mechanical causes of bridge deck cracking. Figure 1 shows the flow chart they created showing the primary factors in the cracking problem.
Figure 1: Causes of bridge deck cracking (Brown, et al., 2001)
24
As seen in this figure, Brown et al. consider shrinkage, thermal stresses, and restraint to be the primary factors in cracking. Later in this literature review, each of these factors will be considered in greater detail.
Shrinkage of concrete is a primary source of strain in bridge decks, and can produce enough strain to crack concrete without additional strain from temperature sources (Krauss and Rogalla, 1996). It is considered by many to be the greatest culprit in the cracking problem (Krauss and Rogalla, 1996).
Temperature effects are the other important source of strain in the concrete matrix. The concrete sets at a specific temperature, locking the matrix to zero temperature stress at that temperature. However, the deck changes temperature, seasonally, daily, from cooling off after the heat of hydration subsides, and from solar radiation on the top surface. These four sources cause significant temperature movement, which occurs according to the coefficient of thermal expansion of the concrete. The stresses induced can both be high and significantly non-uniform (Krauss and Rogalla, 1996).
The final sources of strain are the dead loads and live loads on the structure, along with formwork deflection issues. These strains are less significant, but of concern nonetheless. Several state departments of transportation considered these to be a source of cracking (Krauss and Rogalla, 1996).
25
In an unrestrained system, strain does not cause cracking, but when the system is restrained, the strain translates to stress and causes cracking. The restraint of the deck’s movement converts the strain into stress, according to the modulus of elasticity. Both external and internal sources can provide the restraint. The chief external source is the girders that the deck rests upon. Since the girders will not shrink at the same rate as the deck unless they are cast at the same time of the same material, the girders restrain the deck’s movement. In addition, material differences can cause differential restraint of temperature movements. Internally, rebar, aggregate, and fibers are some of the sources of restraint (Krauss and Rogalla, 1996).
There are several other factors that influence the mechanical cracking problem. Stress relaxation or “creep” of concrete is another key issue, as it is the one factor that can reduce the stresses on the concrete. Altoubat and Lange (2002) analyzed this factor in considerable detail. They found that creep can reduce shrinkage stresses by 50% (depending on the mix design), thus doubling the strain capacity at failure.
Krauss and Rogalla (1996) consider the modulus of elasticity to be another important factor in the cracking problem. The modulus of elasticity of the concrete determines the rate of conversion from strain to stress. Therefore, the stress in the concrete will be higher with a higher modulus of elasticity given the same strain conditions.
The geometry of the bridge deck and girders can also have significant impacts on the cracking behavior of the concrete. Krauss and Rogalla (1996) analyzed different
26
designs analytically and found that the geometry of the deck significantly impacted the shrinkage and thermal strain fields.
Corrosion of reinforcing steel is a well known factor; however, it typically does not become important for several years. Since the present cracking problems usually show up within a year, the corrosion issue will only be considered in passing.
The final factor in the cracking process is the tensile strength of the concrete itself. After the stresses are created by the factors mentioned previously, whether the concrete finally cracks or not is determined by comparing the stress to the tensile strength of the concrete. As shown in Figure 2, both the stress and the tensile strength of the concrete change with time and it is when the stress finally exceeds the tensile strength of the concrete that cracking occurs (Brown et al., 2001 after Mehta, 1993).
Figure 2: Time dependence of restrained shrinkage and creep (Brown et al., 2001 after Mehta , 1993)
27
In summary, the literature indicates that the mechanical process that creates the cracking is as follows. Shrinkage and thermal movement, along with deflections to some extent, put a strain on the deck. This strain would cause no stress if it was unrestrained, but restraint is provided both by the girders and by the reinforcement. This restraint converts some 80% of the strain to stress, depending on the degree of restraint. The actual conversion rate is the modulus of elasticity of the concrete. The creep of the concrete reduces stress by a significant but hard to quantify amount. This stress field is modified by the geometry of the deck, and finally the stress and the tensile strength of the concrete may be compared to see whether cracking is likely to occur. This view of the cracking problem, while probably somewhat simplistic in some areas, gives a reasonable picture of the issues involved in cracking of bridge decks (Krauss and Rogalla, 1996).
Here is a simple example of the mechanics in action, from Krauss and Rogalla
(1996): …If the concrete has a free-shrinkage of 500 microstrain (με), but it is restrained and allowed to shorten only 250 με, the restraint is 50 percent. A concrete with a modulus of elasticity of 4 x 106 psi might have an effective modulus of only 2 x 106 psi, because of creep. The resultant stress would be the product of the strain (500 με) times the restraint (50 percent) times the effective modulus of elasticity (2 x 106 psi) for a resultant tensile stress of 500 psi. If the tensile strength of
28
the concrete is greater than 500 psi, cracking will not occur. However, additional tensile stresses from thermal gradients or loading could crack such a concrete. Therefore, effects of shrinkage and temperature changes, effect concrete modulus, restraint conditions, tensile strength, and loading conditions must be considered. (Krauss and Rogalla, 1996)
Figure 3 shows the factors affecting cracking in bridge decks that are covered in this literature review.
29
Cracking in
Bridge
Decks
Shrinkage Thermal Deflections Restraint Modulus of Creep of Geometry Tensile Corrosion Freeze/ Effects
Elasticity Concrete
Strength Thaw Plastic Heat of Live Loads Internal Rate of Gain Skew Rate of Chloride Air Content (Early Age) Hydration
Increase Permeability
Autogenous Temperature Formwork Reinforce-Depth of Ultimate Rebar Type
at Casting
ment Deck
Drying Cooling after Aggregate Cover
(Long Term) Batching
Carbonation Diurnal Fibers
Cycle
Annual External
Cycle
Solar Girders
Radiation
Heating
Compared Expansion
Joints
With Temp
at Casting
Coefficient of Thermal Expansion
Figure 3: Factors affecting cracking in bridge decks
30 2.1.3 Shrinkage
Shrinkage is thought to be one of the greatest causes of cracking in bridge decks (Krauss and Rogalla, 1996; Phillips et al., 1997). Restrained shrinkage alone can create tensile stresses sufficient to crack the deck. If the deck shrinks 500 microstrain, the deck can easily see tensile stresses exceeding 1000 psi, depending on the material properties and geometric constraints (Krauss and Rogalla, 1996).
There are four types of shrinkage of note. Plastic shrinkage occurs at early age, before the concrete has hardened. This type of shrinkage typically occurs because of poor curing conditions leading to evaporation of water and hence high capillary stresses. Autogenous shrinkage is based on the loss of water due to chemical consumption in the setting chemical reactions, and potentially the actual formation of the crystal structure. Drying shrinkage is the primary long-term shrinkage type, again based upon water loss. Carbonation shrinkage is a long-term shrinkage that occurs when there is a high CO2 concentration in the air around the concrete.
It must be noted that shrinkage as a whole is not well understood. The types of shrinkage can be isolated by using specific tests, but the actual mechanisms by which these shrinkage types proceed are open to argument.
2.1.3.1 Plastic (Early Age) Shrinkage
Plastic shrinkage occurs at early age. It is listed by Issa (1999) as the most important cause of bridge deck cracking. Plastic shrinkage depends on two primary factors: the
31
rate at which surface water forms (bleeding) and the evaporation rate of the surface water (Wang et al., 2001). When the evaporation rate from the top surface of the concrete exceeds the bleed rate at which water rises from the concrete, the top surface dries out. At this point, the free water surface in the concrete drops within the concrete, yielding menisci between the particles. These menisci exert a tensile force due to surface tension on the particles, a suction of sorts. This and a low concrete strength due to top surface desiccation cause cracking (Mindess and Young, 1981; Cheng and Johnston, 1985; Holt, 2001; Brown et al., 2001). Since this type of cracking occurs because of forces near the surface of the concrete, the cracks are typically shallow in depth and originate from the top surface. These cracks, however, are sufficient to assist water and chloride penetration, and to provide stress concentration points for long-term shrinkage cracking. Plastic shrinkage does not require external restraint on the member to create stresses, as the majority of the member is not shrinking, and it is solely the surface that shrinks. Thus, the surface alone will crack. Typical cracks are no more than 2 or 3 feet long and are 2 to 3 inches deep (Xi et al., 2003, Krauss and Rogalla, 1996) and exhibit a typical “turkey track” configuration.
2.1.3.1.1 Curing conditions
Curing conditions are the overriding cause of plastic shrinkage cracking. It is the most common reason cited by transportation agencies for the transverse deck cracking (Krauss and Rogalla, 1996). Curing conditions are blamed by most departments of transportation for the early-age cracking problem. In many cases, the
32
department of transportation’s specifications on bridge deck placement and curing may be ignored, greatly intensifying the problem.
There are several procedures that are important for limiting the plastic shrinkage cracking problem, all revolving around limiting evaporation from the fresh concrete. If possible, the evaporation rate should be measured or estimated, and the evaporation rate limited to 0.20 lb./ft2/hr for normal concrete and 0.10 lb./ft.2/hr. for concrete with a low water to cement ratio (Shing and Abu Hejleh, 1999). Evaporation counter measures are almost mandatory if the evaporation rate exceeds 0.20 lb./ft.2/hr, and cracking is possible even with an evaporation rate of only 0.10 lb./ft.2/hr (Cheng and Johnston, 1985). Nomographs are available to calculate the evaporation rate based on environmental conditions.
33
Figure 4: Accumulation of early age and long term shrinkage, with various curing environments during the first day. Wind = 2 m/s (4.5mph), dry = 40% RH, wet = 100% RH. (Holt, 2001)
Figure 4 gives shows just how significant the curing conditions are in the shrinkage of concrete. Wind can greatly increase the shrinkage of concrete, and the level of wind shown (some 4.5 miles per hour) is often found on a jobsite. Dry conditions (like 40% relative humidity) are similarly commonly found, and proper precautions must be taken to prevent the drying shrinkage shown in the figure from occurring. Interestingly, it has been shown that there is no correlation between curing conditions in the first 24 hours and shrinkage at later times; they are essentially decoupled (Holt, 2001).
Moist curing for an extended period of time is highly recommended (Mindess and Young, 1981). Using a wet burlap system has long been considered the best method,
34
but wind and heat can dry burlap rapidly, necessitating a method for keeping the burlap moist. The moist curing must start within a few minutes of the finishing to get the best results. Fogging during the time between strike-off and the application of the burlap helps reduce early-age plastic cracking as well, and is highly recommended (Xi et al., 2003; Shing and Abu-Hejleh, 1999; Cheng and Johnston, 1985).
Curing compounds can significantly reduce the number of small deck cracks, but this method is not as good as using wet burlap for several days. The film applied is difficult to make continuous, and the moisture from the wet curing aids the strength of the very top of the concrete.
2.1.3.1.2 Consolidation
It has been shown that inadequate consolidation contributes to early age cracking, as well as other issues. Typically, the department of transportation specifications are sufficient to prevent this problem, but are not always carried out in the field.
2.1.3.1.3 Finishing Procedures
Early finishing reduces the size and number of cracks. In addition, double-floated decks seem to have less cracking. In order to allow curing to commence earlier, it is recommended to saw cut the grooving rather than use rake tining of plastic concrete. Rake tining of plastic concrete damages the surface of the hardened concrete. Hand finishing should not be allowed except at the edge of the pavement (Krauss and Rogalla, 1996; Xi et al., 2003; Shing and Abu-Hejleh, 1999).
35
2.1.3.1.4 Mix Design
The mix design of a concrete influences the plastic shrinkage. High water to cement ratios and high cement content increase plastic shrinkage (Aktan et al., 2003; Krauss and Rogalla, 1996). Interestingly, a high water to cement ratio would seem to lead to a higher bleed rate, which according to the accepted model of plastic shrinkage is a good thing. A lower water to cement ratio concrete would probably have its top surface dried out more readily. Early age cracking has become more prevalent as high performance concretes (with a low water to cement ratio) have become more common. Perhaps some further investigation of the relationship of water to cement ratio and plastic shrinkage is in order.
2.1.3.1.5 Admixtures
There are several admixtures that can impact the plastic shrinkage of concrete. Shrinkage reducing admixtures reduce the surface tension of the water in the capillary pores, thus reducing the stress from the pore water. This reduces the plastic shrinkage, but this mechanism also reduces air entraining, which may be problematic. Set retarders can actually increase plastic shrinkage simply by keeping the concrete plastic before setting for a longer period of time (Xi et al., 2003; ACI 212, 1989). Water reducing admixtures can help decrease the shrinkage as well by reducing the water to cement ratio.
2.1.3.1.6 Air temperature
The air temperature at batching directly influences the evaporation rate of the concrete, and thus the plastic shrinkage. It is typically recommended to batch when
36
the air temperature is below 80° F (Xi et al., 2003; Krauss et al., 1995, Shing and Abu-Hejleh, 1999).
2.1.3.1.7 Wind
Several investigators and transportation departments consider wind to be the most significant factor affecting cracking (Krauss and Rogalla, 1996). Wind significantly increases evaporation, which is the main cause of plastic shrinkage cracking (Xi et al., 2003). Most sources recommend setting up temporary wind breaks during casting to limit evaporation until appropriate curing methods can be applied. Some curing procedures are adversely affected with wind, particularly any that have plastic sheeting placed, as the wind can blow under the plastic if the edges are improperly secured. If necessary, casting under a high wind condition should be avoided to reduce plastic shrinkage (Xi et al., 2003; Mindess and Young, 1981).
2.1.3.1.8 Humidity
Humidity decreases evaporation; to increase humidity around the concrete, foggers are often recommended. If the humidity in the air is very low, there can be high evaporation rates even without wind (Xi et al., 2003). More cracking has been observed for concrete cast during low humidities (Krauss and Rogalla, 1996).
2.1.3.1.9 Silica Fume Concrete
Silica fume increases the density of the concrete, decreasing porosity, and thereby also decreasing the bleed rate of the concrete. This inability of water to move within the mix increases the concrete’s susceptibility to plastic shrinkage and plastic
37
shrinkage cracking. It has been shown that silica fume concrete is significantly more likely to crack if improper curing procedures are followed. However, studies have also shown that if appropriate curing procedures are adhered to, the silica fume does not increase plastic shrinkage cracking (Shing and Abu-Hejleh, 1999).
2.1.3.2 Autogenous Shrinkage
Autogenous shrinkage is defined as the macroscopic volume change occurring with no moisture transferred to the exterior surrounding environment, and thus is related to the actual chemical reactions of the concrete. Autogenous shrinkage occurs even when the concrete is completely submersed in water, thus having 100% humidity on the surface. It also occurs even when the surface is made completely air and water proof with some curing agent. Thus its mechanism is not related to surface tension of water at the surface, but rather to the surface tension in pores, a reduction in relative humidity as the pore water is chemically consumed, and the actual volume change from the reactants to the products (Xi et al., 2003; Holt, 2001; Brown et al., 2001; Lura, 2003). The higher performance concretes move the reaction more in favor of lower volume products, increasing the importance of the last mechanism mentioned.
Autogenous shrinkage is usually insignificant compared with plastic and drying shrinkage, but for high-strength concretes with low water-to-cement ratios, it has been shown that autogenous shrinkage becomes important. Most research indicates strength exceeding 6000 psi and water-to-cement ratios below 0.4 are most
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susceptible to autogenous shrinkage (Xi et al., 2003; Holt, 2001; Brown et al., 2001; Lura, 2003).
Autogenous shrinkage is a chemical shrinkage, but not all of the chemical shrinkage translates into autogenous shrinkage, which is an external measurement. Some of the chemical shrinkage ends up as voids in the concrete, as illustrated in Figure 5.
Figure 5: Reactions causing autogenous and chemical shrinkage (Holt, 2001 from Japan, 1999) C = unhydr ated cement, W = unhydr ated water , H y = h ydration pr oducts, and V = voids generated by hydration.
The first source of the chemical shrinkage is from volume reduction of the reaction products. This is dominant at very early age, when the concrete is still liquid. At this age, the chemical and autogenous shrinkage are equivalent. In addition, because the concrete is still liquid, the shrinkage does not result in stress, as the concrete is unrestrained and simply settles.
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After the skeleton of the concrete begins to be formed, there are several mechanisms in play. Figure 6 below illustrates the formation of empty pore volume due to chemical shrinkage, which results in a decrease of the radius of curvature of the menisci and in bulk shrinkage due to increased tensile stresses from the pore water. This is self desiccation shrinkage.
Figure 6: Sc hematic of a c ross-section of hydr ating c ement paste (Jense n and Hansen, 2000). Left: low degree of hydration. Right: high degree of hydration.
Self-desiccation is the most commonly cited mechanism, where the pore water is consumed by the hydration process. As the pores dry, the water menisci in the pores produce significant suction forces on the crystalline structure. Chemical shrinkage is still in play as the chemical reactions proceed and the products of the reaction form. These products are slightly less in volume than the reactants.
There is a third mechanism theorized that relates more to the concrete microstructure and gel formation. Surface tension of the gel particles has been proposed as the mechanism, but it could only be a small part of the autogenous deformation.
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The final mechanism proposed is disjoining pressure, where the adsorption of water to the gel particles is hindered. This occurs where the distance between the solid surfaces is less than two times the thickness of the free adsorbed water layer. The pressure is the result of van der Waals forces, double layer repulsion, and structural forces (Lura, 2003). This pressure is higher at higher relative humidity. When the relative humidity drops from water consumption, the disjoining pressure is reduced, causing shrinkage.
Autogenous shrinkage is hard to reduce without altering the actual water to cement ratio. If the autogenous shrinkage has to be reduced, it has been recommended that 25% of the coarse aggregate be replaced by a water-saturated lightweight aggregate (Xi et al., 2003). Holt (2001) agrees that the water to cement ratio is by far the most important factor in autogenous shrinkage, but lists three other factors that can influence it (shown in Figure 7). Holt was evaluating early age autogenous shrinkage for the most part, but noted three factors: bleed rate, chemical shrinkage, and time to hardening. A higher bleed rated decreases autogenous shrinkage, and earlier hardening does as well. Chemical shrinkage, the volume change when the hydration reaction progresses, directly influences autogenous shrinkage as well, but is generally not under the control of the engineer. Xi et al. (2003) lists these same factors as well.
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Figure 7: Direction of shift in early age autogenous shrinkage when influenced by other factors (Holt, 2001)
2.1.3.2.1 Mix design
Mix design is the factor with the largest influence on autogenous shrinkage. Autogenous shrinkage does not occur unless the water to cement ratio is below 0.42 (Holt, 2001). According to all sources, autogenous shrinkage increases as the water­to-
cement level decreases, particularly below about 0.4 (Shing and Abu-Hejleh, 1999).
2.1.3.2.2 Cement type
Type K cement has a different crystalline structure than standard Portland cements. This shrinkage-compensating cement actually expands as the concrete sets, compensating for other types of shrinkage. Since this occurs inside the concrete, it is an autogenous movement type.
The Ohio Turnpike Commission (OTC) has used type K concrete for many years, and has over 500 bridge decks with type K concrete. The New York Thruway Authority
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(NYTA) cast 47 decks in the early 1990s with this type of concrete. Linford and Reaveley (2004) reviewed the OTC and NYTA for their experiences with type K cement. The OTC has had good experience with type K decks, with most shrinkage cracking eliminated. They had to provide special treatment for the decks, including higher water to cement ratio, faster placement, faster implementation of curing, and continuous wet curing for 7 days. It must be noted that most of these are all well-known techniques for obtaining good shrinkage and cracking results, with or without the type K cement. NYTA had severe problems, and stopped using the cement. Overall, the benefits of type K are debated; some researchers show reduction in cracking, and others showed problems (Xi et al., 2003; Krauss and Rogalla, 1996).
2.1.3.3 Drying (Long Term) Shrinkage
Drying shrinkage is the most significant type of shrinkage in most concrete mixes, and has been called the most deleterious property of Portland cement composites (Zhang and Li, 2001). The mechanisms are similar to those of plastic shrinkage, but occur after the concrete has hardened. Drying shrinkage comes from the transfer of water from the concrete to the surrounding environment, thus increasing the surface tension in the pores. Eventually, the concrete will come to complete equilibrium with the surrounding environment. At that point the movement associated with moisture will simply follow the environmental conditions—if wet, then the concrete swells, if dry, it shrinks (Mindess and Young, 1981).
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There are three mechanisms described in the literature: capillary stress, disjoining pressure, and surface tension. Each of these mechanisms is dominant in a different range of relative humidity. The most important mechanism in field conditions is the capillary stress, which is dominant from 45%-90% humidity. The three mechanisms all appear to be reversible, but a large portion of the drying shrinkage is irreversible. The reason for the irreversibility is not well known; it is thought that the stresses from those three mechanisms cause the calcium silicate hydrate particles to realign to a “matrix stable” configuration. This realignment seems to only occur during the first drying period; after that, subsequent wetting and drying does not have a large impact on the irreversible part of drying shrinkage (Xi et al., 2003; Mindess and Young, 1981; Brown et al., 2001).
It is thought by most researchers that the ultimate shrinkage values are not the most important facet of the drying shrinkage issue. The actual rate of shrinkage is more important, as this compared with strength gain, creep and other time-dependent factors actually determines whether there will be cracking. If the shrinkage occurs quickly while the strength gain occurs slower, the concrete may crack early even though at the fully-developed values of both the concrete would have been strong enough to handle the load. In addition, if the shrinkage occurs quickly, creep is unable to relieve the stress. Xi et al. (2003) cite the following example: “For a concrete prism fully restrained at both ends, cracks may develop at a shrinkage strain of around 200~250 με if not accounting for the creep effect of concrete. Under high shrinkage rate, 200~250 με could easily occur at the age of 10 days under normal
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room temperature and 50% humidity. Therefore, proper measures must be taken to reduce not only the ultimate shrinkage strain but also the shrinkage rate.” It is generally perceived that reducing the shrinkage rate is more difficult than simply reducing ultimate shrinkage.
2.1.3.3.1 Curing methods
Curing of the concrete determines to a large extent the rate at which the drying shrinkage occurs (Krauss and Rogalla, 1996). If the concrete remains in a saturated condition, then drying shrinkage should be nearly eliminated for that period. Thus 7 day wet curing is very beneficial for letting the concrete gain strength before the shrinkage stresses cause cracking, and some even suggest 14 day. However, research by Holt (2001) shows that curing conditions for the first 24 hours do not affect shrinkage occurring at later ages. There seems to be some disagreement over how much curing conditions actually affect long-term behavior.
2.1.3.3.2 Mix Design
Mix design also has a significant impact on drying shrinkage. In particular, decreasing the water content decreases the drying shrinkage of the concrete. Interestingly, this is opposite to the results with autogenous shrinkage. The water to cement ratio has not been shown to have a conclusive effect on cracking, just on shrinkage. Decreasing the cement content decreases shrinkage, as the cement paste itself is the phase that causes the shrinkage. Essentially, high paste volume increases drying shrinkage. Many researchers have noted that high-slump concrete tends to increase cracking, which makes sense: high paste volume increases slump. Schmitt
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and Darwin (1999), for example, recommend that no more than 27% of the total volume of the concrete be cement and water (Schmitt and Darwin, 1999; Linford and Reaveley, 2004; Xi et al., 2003; Krauss and Rogalla, 1996; Cheng and Johnston, 1985).
Krauss and Rogalla (1996) list several other factors known to reduce drying shrinkage: maximizing the amount of aggregate (which reduces paste volume), using Type II cement, and using aggregate with low-shrinkage properties. A soft aggregate, such as sandstone, greatly increases the shrinkage of a concrete over a concrete using a hard aggregate (like dolomite); one researcher showed a 141 percent increase in that case. The absorption of the aggregate has been shown to reflect the drying shrinkage, but a quantitative relationship is not known (Babaei and Purvis, 1995; Cheng and Johnston, 1985). It is also known that cements from different sources can have widely different shrinkage characteristics; in some cases, one cement can have shrinkage over 100% higher than another (Babaei and Purvis, 1995).
2.1.3.3.3 Admixtures
Admixtures can modify the drying shrinkage. Shrinkage reducing admixtures reduce the surface tension in the pore water, reducing the driving force of the drying shrinkage, as well as the other types of shrinkage. Shrinkage reducing admixtures are very effective in reducing drying shrinkage (Xi et al., 2003). High range water reducers, retarders, and superplasticizers seem to have only a minor impact on drying shrinkage.
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2.1.3.4 Carbonation Shrinkage
Carbonation shrinkage occurs when the concrete is exposed to air with high concentrations of carbon dioxide and about 50% relative humidity for long periods of time. The concrete behaves as if it were exposed to drying conditions with a relative humidity far below the actual humidity (Brown et al., 2001). The conditions mentioned above occur most often in structures like parking garages, while bridges seldom have these conditions (Mindess and Young, 1981). Therefore, this type of shrinkage is outside the scope of this work, and will not be discussed further.
2.1.4 Thermal Effects
Thermal effects are as important to the cracking problem as shrinkage is, but are often overlooked since they are largely outside the control of the engineer. Nevertheless, the strain applied by temperature changes alone can easily be enough to cause cracking (Krauss and Rogalla, 1996; Aktan et al.,2003).
The thermal stress-free condition is locked in at the time and temperature of the concrete’s setting. From that time on, any temperature different than that experienced at the setting time will cause strain in the concrete. If this is restrained, then the strain is converted to stress. Differential stresses are created when the deck and the girders of a composite deck are expanding or contracting at different rates.
High early temperatures in the concrete can create early age cracks, as the thermal stresses act upon fresh concrete with low strength. Concretes that have high early
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strength usually also have a high heat of hydration, leading to more thermal cracking problems. To prevent excessive thermal gradients, the peak and placement temperatures of the concrete need to be limited, but how much is open to debate. There are numerous methods to reduce the heat related problems; they are discussed below.
2.1.4.1 Heat of Hydration
The heat of hydration for the concrete sets the baseline upon which all other thermal effects work. A high heat of hydration, combined with an early set time, will lead to an elevated stress-free temperature, which will greatly exacerbate the thermal movement problems. The problems depend also upon the geometry of the member; a large member will retain the heat generated by hydration longer, making a higher temperature when the concrete hardens more likely (Brown et al., 2001). If the concrete sets at, perhaps, 100° F, and the concrete eventually reaches 20° F at some later date, that thermal movement will add over 200 psi of tensile stress to the deck (Krauss and Rogalla, 1996). It is beneficial, therefore, to reduce the heat of hydration and to keep down the temperature at setting.
The heat of hydration is impacted by several factors. The most important is the cement type. A cement heavy in tricalcium silicate will have a much higher heat of hydration than one heavy in dicalcium silicate. Type III cement has the highest heat of hydration, both because of the high tricalcium silicate and tricalcium aluminate percentages, and because the clinker particles are ground to a smaller size, increasing
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their reactivity. Type I cement has a somewhat lower heat of hydration, and Type IV, specially designed to reduce the heat of hydration, has by far the lowest heat of hydration. Typically, the faster the cement gains strength, the higher the heat of hydration, because of the concentration of reactions in time—more reactions at the same time means more heat at that time. It is recommended that cements with a lower hydration heat be used where possible (Xi et al., 2003, Shing and Abu-Hejleh, 1999). In particular, Type II cement, which has slightly lower heat of hydration than Type I, is recommended for general purposes (Krauss and Rogalla, 1996; Shing and Abu-Hejleh, 1999; Babei and Purvis, 1995; Aktan et al., 2003).
For the concrete, however, there are other factors than simply the type of cement. The cement volume in the actual mix design also determines the concrete heat of hydration. Increasing the cement volume in the concrete increases the amount of heat generated by hydration.
Finally, some admixtures alter the heat of hydration. Retarders decrease the maximum heat of hydration by spreading out the hydration reactions in time, giving more time for the concrete to lose heat to the environment. In addition, fly ash has been successfully used to reduce cracking by reducing the strength gain and early concrete temperature (Krauss and Rogalla, 1996; Shing and Abu-Hejleh, 1999).
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2.1.4.2 Temperature at Casting
The actual temperature at the time of set determines the thermal behavior of the concrete from that time forward. Heat of hydration has a large influence on the setting temperature, but so do environmental conditions. The procedures used in the casting of the concrete can significantly modify the setting temperature as well.
If possible, the concrete should be cast at approximately the median temperature for the year; cracking is worse when the concrete is cast at either low or high temperatures (Krauss and Rogalla, 1996; Meyers, 1982; Cheng and Johnston, 1985). Obviously that is rarely possible, but it is possible to bring the temperature of the concrete close to that level. However, the temperature of the concrete at casting is rarely the temperature of the concrete at setting, because the concrete will quickly come to the temperature of the environment (Aktan et al., 2003). For this reason, it is unlikely that procedures such as cooling the mix with nitrogen actually have much impact on the setting temperature.
Agencies usually restrict batching temperature, both of the air and of the concrete itself. Concrete does not set properly at low temperatures; high temperatures cause problems with thermal movement. Air temperature at batching must be between 45° and 80° F (Rogalla et al., 2003). This is not practical in some regions of the country. Concrete mix temperatures must be above 50° F for the first 72 hours, and below 80° F (Xi et al., 2003, Shing and Abu-Hejleh, 1999; Krauss and Rogalla, 1996; PCA,
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1970). This is very difficult to attain if the air temperature is outside that envelope, because concrete quickly approaches the ambient temperature (Aktan et al., 2003).
2.1.4.2.1 Weather
The weather at the time of the concrete setting is important to the temperature of the concrete at setting. It is often recommended to batch late in the day during the summer months; this allows the setting of the concrete to take place late in the evening as the ambient temperature decreases. Night batching has been shown to significantly reduce deck cracking (Krauss and Rogalla, 1996; Purvis, 1989). In the winter, casting should take place so that the concrete will set during the warmest part of the day. These procedures will minimize the effect of the annual temperature cycle on the concrete. It is usually recommended not to batch when the temperature is above 80°.
2.1.4.2.2 Heat of hydration
The heat of hydration, as discussed above, will raise the concrete’s setting temperature. It is rarely feasible for the engineer to modify the mix to reduce the heat of hydration, as strength and shrinkage considerations dictate the mix proportions. Retarders are recommended to reduce the temperature gain from the heat of hydration (Xi et al., 2003).
2.1.4.2.3 Batching Temperature
During the winter and summer, the concrete is often warmed or cooled to meet department of transportation specifications on the temperature of the concrete at
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batching. In the winter, the aggregate is often heated through various means; in the summer, the water is chilled, ice is added, or the mix cooled with liquid nitrogen. Whether this does any good for the actual setting temperature is doubtful. Aktan et al. (2003) found that the concrete temperature at placement had little long term effect because the concrete quickly reached the ambient temperature.
2.1.4.3 Cooling After Batching
The first temperature change that the concrete will see is the actual cooling as the heat of hydration is released. This can very often cause cracking, because the concrete is still weak, but the matrix itself has already formed. The restraint provided by underlying beams and the forms themselves is sufficient to translate the strain into stress. Cracking from this source is usually formed above the uppermost transverse bars and is full depth (Xi et al., 2003).
Krauss and Rogalla (1996) give an example of the potential stress generated by the
cooling of a deck that was 50° F above the temperature of the restraining girders: A 28° C (50° F) temperature change in the deck relative to the girders can cause stresses greater than 1.38 MPa (200 psi) when the concrete has an early effective modulus of elasticity of only 3.5 GPa (0.5 x 106 psi), and greater than 6.89 MPa (1000 psi) when the early effective modulus is 17.2 GPa (2.5 x 106 psi).
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2.1.4.4 Diurnal Cycle
A concrete bridge deck’s temperature will mirror to an extent the ambient conditions of the surrounding environment. The heat of a bridge deck will vary as much as 50° Fahrenheit during the course of a day. This type of thermal movement is too short-term to be alleviated by creep, and thus must be taken by the concrete itself (if restrained). This is the primary source of thermal stress, since the change is non­uniform
on the structure; this non-uniformity is covered in the solar radiation section (Xi et al., 2003; Krauss and Rogalla, 1996).
Krauss and Rogalla (1996) give examples of the levels of thermal stress from the diurnal cycle that can be reached, from analytical analysis of the system. The assumption in these examples is of a linear temperature gradient in the bridge. With a 50° F temperature change, the tensile stresses can reach 1350 psi on simply-supported steel girders, and 1480 psi on simply-supported concrete girders. Over the interior supports of a continuous span bridge, the tensile stress could reach 2000 psi on concrete girders. Those numbers were calculated theoretically from the mechanics of the system; in reality, the concrete would probably fail long before those stresses were reached.
2.1.4.5 Annual Cycle
The annual temperature cycle also brings significant temperature fluctuations to the bridge deck. During a year, the high temperature during a day may go from 0° to 100° Fahrenheit. This type of fluctuation is less problematic, because it is uniform
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across the structure. Thus, the girders and deck will see precisely the same changes. If the deck and girders have the same coefficient of thermal expansion, little stress will be seen. However, when the girders are steel, the total temperature change is the source of the stress, rather than the differential change across the structure (Xi et al., 2003). When combined with the diurnal cycle, the annual cycle brings a temperature range of some 120°, and that is just the air temperature in the surrounding environment. This range is what has to be handled when the deck and girders are not the same material. The concrete itself is also likely to get hotter from radiation—but since that heating is non-uniform and non-linear, it is considered in the next section.
The annual temperature cycle is another of the factors that the engineer has no control over, but it is useful to consider it. Krauss and Rogalla developed equations to calculate the stress developed in a concrete bridge deck with various conditions. Obviously, the worst condition would have the concrete and the girders see different temperatures; if they differ by 50° after the stress-free temperature for the combination is when they are the same temperature, the tensile stress in the concrete can approach 1000 psi, far beyond the tensile capacity of the concrete. However, in most cases the stresses from the annual cycle are limited, since the concrete and steel have at least similar coefficients of thermal expansion (Krauss and Rogalla, 1996).
2.1.4.6 Solar Radiation Heat
This is one of the worst temperature impacts on the bridge deck. The sun heats the top surface, while the bottom surface remains relatively cool, particularly if over a
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large body of water. This yields very significant differential strains, causing curvature and stress in the deck; the free deck will try to curve convex upward. If the solar radiation heating is sustained for a full day, eventually the deck will increase in temperature significantly, while the underlying girders remain relatively cool. This can again put significant stress into the concrete (Krauss and Rogalla, 1996). Figure 8 (Figure 1 from Krauss and Rogalla) illustrates these different types of thermal movements. When these strains are translated to stresses (Figure 9), the stresses can be very large. Figure 9 is also from Krauss and Rogalla, and gives results of a typical calculation. They undertook a large number of similar calculations to determine the maximum stresses that could be seen by the girders and deck.
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Figure 8: Strain effects of various temperature changes (Krauss and Rogalla, 1996)
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Figure 9: Example deck and steel girder stresses for various temperature changes (Krauss and Rogalla, 1996).
2.1.4.7 Compared with temperature at casting
The strain in the concrete depends on the difference between the concrete temperature and that at which the concrete set. The only thing the engineer can control to any degree is the batch temperature, which should be somewhere between the extremes to try to reduce the maximum strains seen.
2.1.4.8 Coefficient of Thermal Expansion
The coefficient of thermal expansion determines how large the strains are with the variation in temperature. This is essentially beyond the control of the engineer.
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However, the differing thermal coefficients of concrete and steel may explain why it has been seen that steel girder bridges are somewhat more prone to cracking than concrete girder bridges. At the time of setting, the stress-free temperature is set, with the concrete usually at a slightly higher temperature than the girders. Then, as the annual and diurnal temperature cycles occur, the concrete deck and steel girders move at different rates, causing stresses to occur in the system.
The coefficient of thermal expansion of concrete is from 4 to 7 με/°F, while that for steel is 7 με/°F (Xi et al., 2003; Shing and Abu-Hejleh, 1999; Mindess and Young, 1981). Concrete with a higher coefficient of thermal expansion is theoretically desirable on a steel girder bridge, in order to match the movement of the girders, but this also would increase the thermal stresses from other sources (like temperature gradients in the deck from radiation), reducing any benefit (Xi et al., 2003).
2.1.4.8.1 Aggregate
The aggregate used has a large impact on the coefficient of thermal expansion. However, it is rarely feasible for aggregates to be chosen based on the thermal expansion coefficient. The final coefficient of thermal expansion is a combination of the coefficients of the cement matrix and that of the aggregate; the paste coefficient is usually 2 to 3 times higher than that of the aggregate (Mindess and Young, 1981; Krauss and Rogalla, 1996; Xi et al., 2003).
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2.1.5 Deflections
This is the third and least important source of strain in the concrete. It, like much of the temperature strain, is of short duration, so the strain cannot be relieved by creep.
2.1.5.1 Live Loads
These obviously produce both stress and strain in the concrete, both after curing and potentially during the curing process if the concrete feels vibrations induced by traffic. These loads are added to those from shrinkage and thermal factors, but it is typically considered that these loads are not significant in the cracking problem. This is because the stresses induced are usually much lower than those from other sources, and they are usually compressive for the deck as well. In addition, these are the loads that the decks are actually designed to carry. Traffic-induced vibrations during curing have not been found to be detrimental (Krauss and Rogalla, 1996).
2.1.5.2 Formwork
The formwork potentially can induce strain, as it is holding the concrete in a certain position during casting. When removed, the structure settles into its dead-load deflected shape, inducing tensile strain in the concrete. There has been some research done on types of formwork, with inconclusive results on whether there is a correlation between formwork type and cracking of the deck. Some advocate stay-in-place forms, while others say they increase the cracking (Krauss and Rogalla, 1996; Cheng and Johnston, 1985). Nothing conclusive has been determined.
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The other type of strain associated with formwork comes from deflection of the formwork while the concrete is plastic. Cracking may occur over the supports of continuous deck bridges in this condition; this situation can be eliminated by using appropriate pour sequences to eliminate formwork deflection inducing tensile stresses in those locations (Krauss and Rogalla, 1996). It should be noted that this type of job sequencing may cause cold joints and construction difficulties.
2.1.6 Restraint
Without restraint, the strain would simply cause movement of the concrete. However, bridge decks are highly restrained systems, both internally and externally. When restraint is present, the strain is converted to stress according to the modulus of elasticity of the concrete (assuming linear elastic behavior). There are two classes of restraints: internal and external. The internal restraint on a bridge deck comes from the reinforcement in the deck, from the aggregate in the deck, and from any fibers in the deck. The external restraint comes from the girders and from any end restraints; the expansion joints are planned to reduce external restraint. However, if the girders and deck are composite, as is often the case, nearly all of the external restraint comes from the girders anyway (Krauss and Rogalla, 1996, Brown et al., 2001).
2.1.6.1 Internal
There are several sources of internal restraint to the concrete matrix. The reinforcing steel is chosen to carry load, but it also is a restraint to the concrete. When the concrete shrinks, the reinforcement does not, thus inducing tensile stress in the concrete and compressive stress in the reinforcement.
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2.1.6.1.1 Reinforcement
The rebar imbedded in the concrete provides a significant degree of longitudinal restraint, and to some extent lateral as well. Since the loads are most significant longitudinally, where they can accrue along the length of the bridge, this is a problem for the bridge deck. Embedded reinforcement, to a lesser extent than girders, restrains the deck against shrinkage and thermal movement, as the coefficient of thermal expansion of the reinforcement is likely different from that of the deck. Of course, the engineer cannot remove the reinforcement from the deck, but there are a few factors that are under the engineer’s control.
2.1.6.1.1.1 Epoxy coated
Epoxy coated rebar behaves differently in its interaction with concrete than does standard rebar. It has been shown that bridges with epoxy-coated rebar behave worse than those with standard black rebar. There is an increasing likelihood for cracking shown, and the epoxy-coated bars develop considerably less bond stress. The cracks tend to be larger with the epoxy-coated rebar (Krauss and Rogalla, 1996; Meyers, 1982). The epoxy rebar helps chloride-ion protection in the laboratory under ideal conditions, but in practice there has not been any benefit found. In addition, the epoxy sometimes delaminates from the steel, causing a failure zone to develop at the bonding surface (Linford and Reaveley, 2004).
2.1.6.1.1.2 Rebar location
Some researchers felt like the rebar location, particularly how much cover was present, had an impact on the cracking. It has been shown that cracking tends to
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occur over the transverse reinforcing steel. It is possible that this occurs because of insufficient cover at those locations. As the concrete settles in the plastic phase, a zone of weakness tends to develop over the rebar, which fractures first under the stresses leading to cracking (Aktan et al., 2003; Issa, 1999; Linford and Reaveley, 2004; Babaei, 2005).
2.1.6.1.2 Aggregate
It has been shown that the aggregate types have a significant impact on all facets of concrete behavior. Aggregate provides a large measure of the concrete’s internal restraint. However, it is rarely feasible to choose aggregate types based upon the measure of internal restraint provided. Aulia (2002) demonstrated that the type of aggregate had a significant impact on the properties of the concrete.
Clean, low shrinkage aggregate is important in getting a high quality concrete. It is well known that the type of aggregate has a significant impact on shrinkage of the concrete, and on the time to crack as well (Krauss and Rogalla, 1996).
Larger aggregate is recommended in a number of sources, in order to minimize the paste volume without sacrificing workability (Xi et al., 2003, PCA, 1970; Shing and Abu-Hejleh, 1999). In addition to minimizing paste volume, the larger aggregates tend to bear directly on one another, so shrinking paste cannot move them. This tends to channel the stress into microcracks within the cement paste, rather than shrinkage. As long as these microcracks to not turn into larger cracks, the effect is considered
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beneficial. It is commonly recommended to achieve the highest possible aggregate volume in the mix, as less paste decreases shrinkage and thermal problems (Xi et al., 2003). “In general, concrete mixes with good quality, clean, low shrinkage aggregate with high aggregate to paste ratio have been observed to perform better (Saadeghvaziri and Hadidi, 2002).”
2.1.6.1.3 Fibers
Fibers provide internal restraint as well, particularly against movement before curing. Steel fibers will continue to provide restraint after curing, as their high modulus of elasticity will continue to take load. Polymer fibers stop providing restraint once the concrete’s modulus of elasticity becomes higher than the fibers’. There is some question whether early restraint is beneficial or detrimental to the concrete. If the concrete is still in the plastic stage, there would not be any stress captured in the matrix, so it likely doesn’t hurt to have this early restraint.
2.1.6.2 External
The external restraint on bridge decks is also significant. The girders are the primary source of the restraint. The best-case scenario is if the girders and deck are cast monolithically; then the shrinkage stresses are equal, and the thermal effects are minimized as well (Krauss and Rogalla, 1996). Most bridges, however, have the deck cast independently from the girders, and are composite systems.
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2.1.6.2.1 Girders
The girders are the portion of the bridge in contact with the deck, and thus their composition and design can influence the behavior of the bridge deck. As the deck contacts the girders all along the length of the deck, and shear systems such as shear studs are used, longitudinal movement of the deck relative to the girders is prevented. Girders restrain the deck movement whenever they do not have temperature or shrinkage strains identical to the deck. Because steel girders do not experience any long term drying shrinkage, they tend to exert greater restraint on the deck than concrete girders. Since only a portion of the deck is restrained, there are induced stresses from the eccentric restraint present as well (Krauss and Rogalla, 1996).
When large girders are used, they can restrain approximately 60% of the uniform free strain at the upper surface of the deck; smaller girders can restrain 35 to 45% of the free strain at the upper surface (Krauss and Rogalla, 1996). Of course, there are many other variables as well.
If the deck has a linear free strain rather than a uniform free strain, the deck tries to curve to alleviate this. This type of movement is restrained at a much higher percentage, from 75 to 95% (Krauss and Rogalla, 1996).
2.1.6.2.1.1 Concrete vs. steel
Due to the fact the steel has a different coefficient of thermal expansion than concrete, the degree of restraint placed by differential movement depends on the
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material of the beams. In addition, the steel has a higher modulus of elasticity, leading to a higher degree of restraint on any free strain in the deck. Finally, the steel girders do not shrink like the concrete deck; the concrete deck strain is completely restrained by the girders. This, combined with the thermal difficulties, explains why cracking is more common on steel girder structures (Xi et al., 2003, Aktan et al., 2003; Krauss and Rogalla, 1996; Meyers, 1982; Cheng and Johnston, 1985; Linford and Reaveley, 2004).
2.1.6.2.1.2 Continuous vs. Simply-Supported
It is thought that continuous-span structures are more susceptible to cracking than simple-span structures (Krauss and Rogalla, 1996; Meyers, 1982; Aktan et al., 2003; Linford and Reaveley, 2004). This is likely due to the negative moment regions over supports and to the longer stretches of deck without any expansion joints. The negative moment regions induce tension in the deck over the support, which a deck already in tension due to shrinkage and potentially thermal effects is ill-prepared to withstand (Cheng and Johnston, 1985; Perfetti et al., 1985).
2.1.6.2.1.3 Girder size and spacing
Research indicates that the size and spacing of the girders effect cracking, but as these are designed based on other issues, they cannot be altered simply to protect the bridge deck. Restraint is increased with larger girders, and with more girders; higher restraint increases the likelihood of cracking (Shing and Abu-Hejleh, 1999).
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2.1.6.2.1.4 Composite deck/girder systems
Composite decks and girders are the norm in bridge design, as they greatly improve the efficiency of the load-carrying system. Most of the discussion of restraints thus far has assumed that the deck and girders act compositely. However, these systems are the source of much of the restraint upon the system. If the deck and girders did not act compositely, the deck would be free to move with shrinkage and thermal strains to a much greater extent (Krauss and Rogalla, 1996). It is not a coincidence that the cracking problem became much more pronounced as the use of a composite deck/girder system became common.
However, it would be premature to advocate the return to noncomposite systems. Further research into the relative merits of the systems is in order, however, particularly in light of the high cost of repairing and replacing cracked decks.
2.1.6.3 Expansion joints
The design and placement of expansion joints can affect how well movements are taken up by the bridge, but they cannot alleviate restraint placed on the deck by the simple presence of the girders.
2.1.7 Modulus of elasticity
The modulus of elasticity of concrete is poorly understood, in that the modulus of concrete changes both over time and with loading. According to Krauss and Rogalla (1996), the modulus of elasticity affects the stresses in the concrete more than any other property. The modulus of elasticity determines the conversion ratio of strain to
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stress in the concrete (Xi et al., 2003). As the strain is the given for both shrinkage and thermal movements, a lower modulus of elasticity will decrease the stress in the concrete. However, a lower modulus of elasticity comes from a concrete with a lower binder ratio, and thus usually a lower strength as well.
A concrete’s modulus of elasticity approximately mirrors the concrete’s strength (Xi et al., 2003). It is unclear if there is any net benefit from reducing the binder ratio, since the strength is usually reduced. Of course, the external loads apply a given stress to the system, so a lower modulus of elasticity will increase deflections--except that the effect will simply be a reduction of the load taken by the deck and an increase of the load taken by the girders (whose modulus of elasticity is a constant).
To reduce the modulus of elasticity without reducing the strength, the primary approach is to use aggregates with a low modulus of elasticity (Xi et al., 2003; Krauss and Rogalla, 1996). Aulia (2002) also found that the modulus of elasticity was largely dependent on the aggregate used, and demonstrated that the relationship held true in fiber-reinforced concrete as well. Whether choosing aggregate to give a low modulus of elasticity is practicable depends on the location where the concrete is batched.
2.1.7.1 Modulus gain
There is some research done of the modulus gain curves. These curves essentially mirror the strength-gain curves of the concrete. In order to get better crack
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performance, Xi et al. (2003) recommend that a concrete with low early strength and modulus of elasticity be used. However, the cracking performance depends on the relationship of tensile strength to the modulus of elasticity, and that relationship is very hard to determine, so attempting to avoid cracking by using a low modulus concrete may not succeed.
2.1.8 Creep of Concrete
Creep of concrete is one factor beneficial to the engineer. Creep occurs with when the concrete is under load for long periods of time. Over time, the concrete slowly moves away for the load, deforming according to the load. Essentially, concrete tries to alleviate stress by a restructuring of the matrix. There are two types of creep: basic creep, which occurs without moisture movement to or from the environment, and drying creep, which is the additional creep caused by drying. The differences between these types of creep, and the fact that there is no distinct separation between instantaneous strain and time-dependent strain, make quantifying creep difficult (Linford and Reaveley, 2004). Research has been done on how great a benefit can be expected from creep and what influences its behavior. Krauss and Rogalla (1996) list creep as one of the major factors effecting bridge deck cracking. Creep occurs in the cement paste; aggregates do not creep. However, lower modulus aggregates encourage creep, possibly by increasing the localized stress in the cement paste (Xi et al., 2003). The nature of creep itself is not well understood; the mechanism seems to be related to the response of calcium silicate hydrate to stress—calcium silicate hydrate has multiple phases it may switch between (Mindess and Young, 1981).
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It has been shown that the tensile creep can relax shrinkage stresses by up to 50%, doubling the strain failure capacity. Both the magnitude and time history of the shrinkage stress influence the time of cracking. Altoubat and Lange (2002) showed that the tensile creep caused their sample mixes to crack at twice the expected failure time based on shrinkage analysis for high performance concrete, and three times the expected failure time for the standard mixtures. Interestingly, they found that the actual evolution of the stress greatly altered the creep behavior. Concrete in a restrained shrinkage test that was sealed for three days and then unsealed actually cracked earlier than unsealed concrete. This, they believe, comes from the higher modulus of elasticity of the sealed concrete, and the exposure shock acceleration of the shrinkage. In addition, they showed that periodic wetting increased the creep of the concrete.
The creep of concrete typically mirrors the compression strength of the concrete. The creep rate (the concrete’s rate of relaxation) decreases at a faster rate than the modulus of elasticity and tensile strength increases. This allows the stress into the concrete to catch up to the tensile strength over time (Figure 10). Note the tensile strength curve is flatter than the stress gain curve (Brown et al., 2001).
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Figure 10: Time depe ndence of res trained shrinkage, creep, and tensile strength (Brown et al., 2001 after Mehta, 1993)
2.1.8.1 Mix Design
There has been research done on exactly what types of mixes creep more or less. In particular, concrete with higher water content creeps more (Krauss and Rogalla, 1996). Since higher water content also increases shrinkage, it is unclear whether this addition of water is actually beneficial. Increasing cement paste volumes increase the creep potential (Xi et al., 2003).
As the compressive strength of a concrete increases, creep decreases and tensile strength increases. However, the creep decreases at a much greater rate than the increase of the tensile strength. This helps to explain why higher strength concretes usually have worse crack performance than normal strength concretes (Xi et al., 2003).
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2.1.8.2 Curing Conditions
Curing conditions significantly modify the creep behavior of concrete. Drying creep dominates basic creep (creep not depending on air drying) on bridge decks, which are usually drying from both sides. “Drying creep is typically 2 to 3 times basic creep when the air relative humidity is 70 to 50 percent, respectively (Krauss and Rogalla, 1996).”
2.1.8.3 Admixtures
Addition of retarders can increase the creep at early age, which can relieve more of the early age shrinkage and thermal issues. Slower curing mixes have higher creep (Krauss and Rogalla, 1996).
2.1.8.4 Plastic Settlement
Plastic settlement of concrete occurs while the concrete is still fresh. As water rises to the surface, the concrete subsides. If there is insufficient cover, cracking will occur over the top reinforcement as the concrete subsides on either side. Babaei (2005) considers this one of four primary causes of bridge deck cracking.
2.1.9 Geometry
The geometry of the design can influence bridge deck cracking, as it can influence stress concentrations and differential movements. This is a very complex subject, and thus difficult to make generalizations about, but a few things are known about how geometry influences bridge deck cracking.
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2.1.9.1 Skew
Some respondents in the survey indicated that they thought skew increased cracking, probably because of stress concentrations. Krauss and Rogalla (1996) believe that skew does not significantly affect transverse cracking, but that it does cause slightly higher stresses near the corners. One researcher (Purvis, 1989) found bridges with a skew over 30 ° were more susceptible to transverse cracking.
2.1.9.2 Depth of Deck
The depth of deck influences the differential movements associated with solar radiation heating of the top surface and can also influence other temperature effects, as the inner core will retain heat longer. However, for actual concentration of stresses, the depth of deck has a minimal impact. Though research is lacking, the information that there is indicates that thinner decks lead to more cracking (Xi et al., 2003; French et al., 1999).
2.1.9.3 Cover
It is believed that the concrete cover does have an impact on deck cracking, but there is not a consensus on what that impact is. Shallow cover increases the likelihood of settlement cracking (Krauss and Rogalla, 1996; Cheng and Johnston, 1985). However, if the cover gets too deep, over about 3 inches, the steel reinforcement is less effective at distributing tensile stresses (Krauss and Rogalla, 1996). Some researchers found worse cracking with cover over 3 inches while others found no correlation. Top cover between 1.5 and 3 inches is recommended (Xi et al., 2003; Krauss and Rogalla, 1996; PCA, 1970).
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2.1.10 Tensile Strength
The tensile strength of the concrete determines if the concrete will actually crack. Unfortunately, concrete is very weak in tension and the actual tensile strength is poorly understood, as it changes with time. The tensile strength of concrete is often estimated as 10% of the concrete’s compressive strength (ACI Committee 318, 2002). The actual tensile strength is subject to considerable fluctuation from sample to sample, because the tensile strength is very sensitive to anything acting as a stress concentrator or crack initiator. Once the concrete starts cracking in tension, it fails almost instantly.
The concrete cracks when the stress is higher than the tensile strength at that time. If the stresses develop faster than the strength, the concrete will crack at early age. Figure 11 shows the tensile strength curve—when the stress reaches the tensile strength, the concrete will crack.
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Figure 11: Time depende nce of res trained shrink age, stress relaxation (cree p), and tensile strength (Brown et al., 2001 after Mehta, 1993)
To further complicate matters, some evidence shows that the concrete cracks below its tensile strength. Table 1 shows some results obtained by Altoubat and Lange (2002) showing that the concrete was cracking at a restrained shrinkage stress below that of the direct tensile strength. Likely this would be due to the likelihood of flaws in larger samples causing cracking to propagate at a lower stress level.
Table 1: Restrained shrinkage stresses and age at cracking (Altoubat and Lange, 2002)
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There are two important factors: the rate of increase and the ultimate strength. If the tensile strength of the concrete rises at a fast enough rate, it can outpace stresses induced by shrinkage at early age, preventing cracking at early age. Long term, the ultimate tensile strength needs to be high enough to resist all stresses that come upon it. There are several factors that can increase tensile strength.
2.1.10.1 Fibers
Fibers can greatly help tensile strength at early age. However, polymer fibers have a modulus of elasticity lower than that of hardened concrete, and thus do not help long term. It has been shown that steel fibers increase ultimate tensile strength. The fibers are potentially very beneficial in increasing the rate of increase of the tensile strength, thus avoiding early age cracking (Kao, 2005).
2.1.10.2 Mix Design
A stronger concrete will have a higher tensile strength. Thus, lower water to cement ratios, higher cement contents, and other factors that are known to increase concrete compression strength will also increase the tensile strength. Unfortunately, these factors usually also increase shrinkage and thermal problems, so if trying to limit cracking, often it is not beneficial to increase the concrete’s strength.
2.1.11 Corrosion
Corrosion is often a long term cracking problem. Much of the corrosion problems come from having existing cracks that allow ingress of water and salts. These cracks accelerate the corrosion problem, which increases the cracking problem.
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2.1.11.1 Chloride Ion Penetration
Different types of concrete corrode at different rates, depending on the permeability of the concrete and the degree of passivation. Silica fume has been added to increase the density of the concrete, but many researchers indicate that silica fume increases sensitivity to curing procedures. If the concrete is cured properly, cracking can be avoided for the most part (Shing and Abu-Hejleh, 1999). Silica fume has a high heat of hydration, is sticky, and is expensive; these issues tend to negate the benefits in ion penetration (Xi et al., 2003).
2.1.11.2 Rebar Type
Epoxy-coated rebar has not been shown to reduce the corrosion problem in the field. In the lab, it performs well, but that is under ideal conditions. After handling in the field, the epoxy has shown both delamination and scratching. Epoxy-coated rebar recovered from failed structures often show delamination and corrosion problems. Epoxy rebar tends to localize the corrosion, increasing the rate of corrosion at those places. It has been shown that cracks tend to larger in bridge decks with epoxy-coated rebar (Krauss and Rogalla, 1996; Meyers, 1982; Linford and Reaveley, 2004).
Stainless-steel rebar does not corrode, but it is very expensive and has only been used by one Department of Transportation, Oregon’s. Stainless-clad rebar seems to be a viable alternative, as it costs only some 50% more than standard rebar and shows significant resistance to corrosion.
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2.1.12 Department of Transportation Opinions
Many papers have been published that include results of surveys on the causes of bridge deck cracking. In addition, many Departments of Transportation commissioned researchers to evaluate what the causes of bridge deck cracking were in their state. These causes may be mechanical, procedural, or a number of other things. A brief review of the surveys and opinions of the reports are presented here.
Krauss and Rogalla (1996) surveyed 52 agencies in the United States and Canada. Most of the respondents indicated that they considered early transverse cracking a problem; nearly all report extensive cracking on bridge decks. The agencies were requested to indicate what they thought to be the causes of bridge deck cracking. Table 2 gives the results of that question; the number in parentheses is the number of responses giving that cause.
Twenty agencies (out of fifty-two) consider improper curing to be a cause of cracking. Wind, thermal effects, and air temperature were each listed by seven agencies. These cannot be remediated easily, but correcting the curing problems should be a high priority. The most common materials problem cited was concrete shrinkage, with drying shrinkage specifically singled out. A few of the agencies also considered deflection design to be a reason for cracking in bridge decks.
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Table 2: Causes of bridge deck cracking, agency survey (Krauss and Rogalla, 1996)
Construction
Materials
Design
improper curing (20) wind (7) thermal effects (7) air temperature (7) relative humidity (4) vibration (2) placement conditions/weather (2)
concrete shrinkage (17) [5 cited drying shrinkage specifically] concrete mix design (7) plastic shrinkage (3) excessive cement (3) concrete temperature (3) use of retarders (2)
deflections (7) excessive cover (3) placement sequence (2)
In addition, Krauss and Rogalla ranked the causes of bridge deck cracking they evaluated from their own research and many other sources. Table 3 gives those findings. These findings are very similar to those discussed earlier throughout the analysis of the mechanical causes of cracking.
78 Table 3: Factors affecting bridge deck cracking (Krauss and Rogalla, 1996)
79 The Kansas Department of Transportation recommended that a silica fume overlay be used to decrease permeability. In addition, wet cure specifications were recommended. They used wet burlap for 7 days, and it cut cracking by 50%. Finally, they liked polymer overlays, but recommend a heavy grit blast (#6 or #7) (Xi et al., 2003).
According to the Utah Department of Transportation report (Linford and Reaveley, 2004), there are a number of factors causing cracking. Restrained shrinkage is listed as the most common cause. Issa (1999) suggests ten causes, listed in order of descending importance:
1.
Inadequate concrete curing procedures which result in high evaporation rates and thus a high magnitude of shrinkage, especially in early age concrete.
2.
The use of high slump concrete.
3.
High water-to-cement ratios due to inadequate mixture proportions and retempering of concrete.
4.
Insufficient top reinforcement cover.
5.
Inadequate vibration of the concrete.
6.
Deficient reinforcing details of the joint between a new and old deck.
7.
Sequence of deck section placement.
8.
Vibration and loads from machinery.
9.
The weight of concrete forms.
10.
Deflection of formwork.
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The Utah Department of Transportation analysis of their bridges found that composite deck attachment to girders, bents, diaphragms, and abutments exacerbated the cracking problem, as it increased the restraint of the deck. Steel girders, as opposed to concrete girders, greatly increased the cracking problem; this is probably because of the differences in thermal expansion coefficients or the difference in thermal mass. Large concrete placements also increase cracking.
The Michigan Department of Transportation report (Aktan et al., 2003) included analysis of a database of inspections. They had several conclusions:
•
More cracks were observed on the continuous bridges than the simple span bridges.
•
Bridges with PCI (Precast Prestressed Concrete Institute) girders showed less longitudinal crack density than other girder types.
•
More transverse and diagonal cracks were observed on bridges with adjacent box girders than other girder types.
•
Map cracking was only observed on bridges with steel girders.
Xi et al. (2003) conducted an analysis of Colorado bridges for the Colorado Department of Transportation, and developed a list of recommendations as well. They recommended:
•
Type II cement or Type I cement with increased fly ash.
•
Cement content below 470 lb/yd3 if possible.
•
Water to cement ratio around 0.4.
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•
At least 20% Type F fly ash.
•
Maximum 5% silica fume.
•
May use ground granulated blast furnace slag to improve durability.
•
Specify strength at 1, 3, 7, 28, and 56 days.
•
Consider using permeability, drying shrinkage, and crack resistance tests as acceptance tests.
•
Largest aggregate size possible and well graded aggregate to minimize cement
paste volume. In addition, they recommended a number of things regarding design factors, primarily aimed at minimizing restraint. For construction practice, it is recommended that the air temperature be between 45° and 80° F for batching, and generally to reduce evaporation however possible. They recommended 7 day continuous moist curing. (Shing and Abu-Hejleh,1999; Xi et al., 2003)
The Michigan report (Aktan et al., 2003) gives the responses of thirty-one Departments of Transportation in regards to the causes of bridge deck cracking. Each respondent was asked to give the three top causes of bridge deck cracking in their jurisdiction. Figure 12 gives the responses.
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What are the top three causes of early age bridge deck cracking in your jurisdiction?
results of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curingresults of a poll for the frequency top three causes early age bridge deck cracking from aktan et. al. 2003 leading cause polled is substandard curing
Substandard Thermal Restraint Mix Design Structural Epoxy Rebar Construction Other Curing Stress System Practice
Causes
Figure 12: Frequency of top three causes of early-age bridge deck cracking (Aktan et al., 2003)
Research in the U. K. has indicated that their early age cracking problem is primarily due to restraint of early thermal movement, rather than restraint of shrinkage as previously thought (The Highways Agency, 1989). The researchers note that cracking has become more prevalent in recent years, as higher strength concretes have been implemented; higher strength concretes usually also produce more heat in the curing period. Thermal movement would be of little consequence if the member was unrestrained, but bridge decks are highly restrained by the beams on which they rest. In plain concrete, thermal cracking tends to yield a few wide cracks; minimal temperature reinforcement leads to more and smaller cracks.
Babaei (2005) reduced all the causes of bridge deck cracking to four central points: settlement of plastic concrete, thermal shrinkage of curing concrete, drying shrinkage
83 of hardened concrete, and flexure. The causes for each of these mechanical processes are then identified and possible methods for reduction given.
Plastic settlement occurs as the concrete bleeds. Often, voids develop under transverse reinforcement bars where rising water collects, and a crack develops above, due to the restraint upon settlement at that location. Several factors promote this condition: shallow cover, a higher slump mix, and large reinforcement size. Babaei constructed a table showing the probability of cracking based on these conditions (Table 4).
Table 4: Probability of Plastic Shrinkage Cracking (Babaei, 2005)
Probability of Cracking (percent)
2 in. slump
3 in. slump
4 in. slump
Bar Size
#4
#5
#6
#4
#5
#6
#4
#5
#6
¾ in. cover
80%
88%
93%
92%
99%
100%
100%
100%
100%
1 in. cover
60%
71%
78%
73%
83%
90%
85%
95%
100%
1.5 in. cover
19%
35%
46%
31%
48%
59%
44%
61%
72%
2 in. cover
0%
2%
14%
5%
13%
26%
5%
25%
39%
Thermal shrinkage during curing is another major type of problem. The concrete cures at high temperature from the heat generated by hydration. It then cools, but is restrained from shrinking by the beams, causing stresses in the deck. Cracking thus occurs as the deck cools.
Babaei states that the difference between the deck and beam temperature contributes strain at the rate of about 5.5 microstrain/degree F. Creep cannot compensate,
84 because the stresses are fully realized within a few days. A temperature differential of about 40 degrees F is enough to produce cracking without other factors; other factors such as drying shrinkage contribute to cracking with less temperature differential. It is best, therefore, to keep the differential to 22 degrees F or less. To do this, less cement, Type II cement, or retarders are recommended. In addition, precautions should be taken in cold weather.
Drying shrinkage cracking is the third type of problem addressed by Babaei. This occurs over long periods of time, on the order of a year. Assuming that creep is 50% of shrinkage, 400 microstrains of drying shrinkage would be needed to crack the concrete. An 8 to 9 inch thick deck can shrink up to about 550 microstrains, depending on the mix. The deck shrinkage is about 2.5x less than that of standard ASTM shrinkage prisms. Therefore, a reasonable parameter for maximum long term specimen shrinkage (assuming deck/beam thermal differential of 22F) would be about 700 microstrains. For 28 day shrinkage, that number would be about 400 microstrains.
There are several factors affecting drying shrinkage cracking mechanically. Aggregate mineralogy is one; porous, “soft” aggregate concrete can have shrinkage twice that of concrete with hard, non-porous aggregates. The type and source of cement also impacts shrinkage; it is best to use cement from a proven source, and type II if possible. If admixtures are used, it is important to test the mix beforehand
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in case unforeseen interactions occur. Finally, minimizing the water in the concrete is key.
The final primary cause of cracking in the opinion of Babaei is from flexure, particularly from unshored construction in continuous bridge decks. To minimize early cracking from this source, it is best to place the deck concrete in midspan first. This minimizes the movement in the area over the support after that section is placed. (Babaei, 2005)
It appears, then, that the causes of cracking are many and varied. Design, construction, and materials issues are all considered contributors. Many point to curing problems as a primary cause of cracking. A large proportion point to shrinkage problems associated with the mix design. A number of design issues seem to be neglected as well, though often designs are non-negotiable in most aspects. It seems that thermal problems are largely ignored. The number of causes is large, and a number of actions not common in construction could help reduce cracking.
2.1.13 Application in the Field
The Michigan Department of Transportation report (Aktan et al., 2003) gave an interesting commentary on the code and adherence to the code by the contractors. Construction monitoring of projects was conducted to see whether contractors adhered to the MDOT Standard Specifications for Construction. There were a number of areas that did not meet the standards:
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•
Freefall of the concrete was often more than 6 inches.
•
Vibratory compaction was often not done within 15 minutes of placing, as concrete delivery delays sometimes exceeded 30 minutes.
•
Vibrators were not used in a pattern, but rather randomly. Vibrators seemed to be used to move concrete into place.
•
Curing was applied for 7 days, but proper precautions were not taken to ensure it was a wet cure operation (which was required).
•
Curing compound was applied very late, rather than immediately after bleed water had left. Sometimes the entire deck was placed before curing compound was applied.
•
Far more than the maximum of 10 feet of textured concrete were left exposed without curing compound.
•
Burlap was not applied until the next day, and then not properly wetted. It was supposed to be placed as soon as the concrete surface could support it, not more than two hours after pouring.
•
Proper procedures for keeping burlap wet were not followed; no soaker hoses were used.
•
The expansion joint boundaries are problematic. Excess concrete overflows, loses its plasticity, and is scraped off and thrown in with the deck concrete near the joint. Concrete that falls off the joints should not be placed back on the deck. (Aktan et al., 2003)
Thus, it appears that even if the departments of transportation have appropriate specifications in place for curing and other construction issues, these specifications
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are not always followed. In design, deck cracking problems are generally ignored as a design parameter. Concrete mix designs are usually created to maximize strength and other parameters such as freeze-thaw resistance, but shrinkage and crack resistance are generally relegated to secondary consideration.
2.1.14 Summary/Conclusion
The United States has a vast bridge deck cracking problem, which has grown in recent years with the increasing use of high strength concrete and the commonplace usage of composite girder/deck designs. There are several key improvements that can help improve the cracking problem.
This literature review has discussed the mechanics of bridge deck cracking. Many causes of bridge deck cracking were identified, but not all are under the control of the engineer. Figure 13 attempts to illustrate the areas where the engineer has good control of the causes of cracking. Many aspects of the bridge design are controlled by the geometry and loads, so the engineer has only minimal control. Some areas, like the thermal movement, are environmental conditions. There are several key areas where the engineer has good control: plastic and drying shrinkage of the concrete deck, the restraint in the deck provided by fibers, and the rebar type used. With these, and making good choices where only moderate control is possible, cracking can be controlled.
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Cracking in
Bridge
Decks
Factors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint, modulus of elasticity, creep, geometry, tensile strength, corrosion and freeze-thawFactors affecting cracking in bridge decks. These include shrinkage, thermal effects, restraint,